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Network Working Group B. Fraser Request for Comments: 2196 Editor FYI: 8 SEI/CMU Obsoletes: 1244 September 1997 Category: Informational

                       Site Security Handbook

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.


 This handbook is a guide to developing computer security policies and
 procedures for sites that have systems on the Internet.  The purpose
 of this handbook is to provide practical guidance to administrators
 trying to secure their information and services.  The subjects
 covered include policy content and formation, a broad range of
 technical system and network security topics, and security incident

Table of Contents

1. Introduction……………………………………………. 2 1.1 Purpose of this Work…………………………………….. 3 1.2 Audience……………………………………………….. 3 1.3 Definitions…………………………………………….. 3 1.4 Related Work……………………………………………. 4 1.5 Basic Approach………………………………………….. 4 1.6 Risk Assessment…………………………………………. 5 2. Security Policies……………………………………….. 6 2.1 What is a Security Policy and Why Have One?………………… 6 2.2 What Makes a Good Security Policy?………………………… 9 2.3 Keeping the Policy Flexible………………………………. 11 3. Architecture……………………………………………. 11 3.1 Objectives……………………………………………… 11 3.2 Network and Service Configuration…………………………. 14 3.3 Firewalls………………………………………………. 20 4. Security Services and Procedures………………………….. 24 4.1 Authentication………………………………………….. 24 4.2 Confidentiality…………………………………………. 28 4.3 Integrity………………………………………………. 28

Fraser, Ed. Informational [Page 1] RFC 2196 Site Security Handbook September 1997

4.4 Authorization…………………………………………… 29 4.5 Access…………………………………………………. 30 4.6 Auditing……………………………………………….. 34 4.7 Securing Backups………………………………………… 37 5. Security Incident Handling……………………………….. 37 5.1 Preparing and Planning for Incident Handling……………….. 39 5.2 Notification and Points of Contact………………………… 42 5.3 Identifying an Incident………………………………….. 50 5.4 Handling an Incident…………………………………….. 52 5.5 Aftermath of an Incident…………………………………. 58 5.6 Responsibilities………………………………………… 59 6. Ongoing Activities………………………………………. 60 7. Tools and Locations……………………………………… 60 8. Mailing Lists and Other Resources…………………………. 62 9. References……………………………………………… 64

1. Introduction

 This document provides guidance to system and network administrators
 on how to address security issues within the Internet community.  It
 builds on the foundation provided in RFC 1244 and is the collective
 work of a number of contributing authors. Those authors include:
 Jules P. Aronson (, Nevil Brownlee
 (, Frank Byrum (,
 Joao Nuno Ferreira (, Barbara Fraser
 (, Steve Glass (, Erik Guttman
 (, Tom Killalea (, Klaus-
 Peter Kossakowski (, Lorna Leone
 (, Edward.P.Lewis
 (, Gary Malkin (,
 Russ Mundy (, Philip J. Nesser
 (, and Michael S. Ramsey
 In addition to the principle writers, a number of reviewers provided
 valuable comments. Those reviewers include: Eric Luiijf
 (, Marijke Kaat (, Ray Plzak
 ( and Han Pronk (
 A special thank you goes to Joyce Reynolds, ISI, and Paul Holbrook,
 CICnet, for their vision, leadership, and effort in the creation of
 the first version of this handbook. It is the working group's sincere
 hope that this version will be as helpful to the community as the
 earlier one was.

Fraser, Ed. Informational [Page 2] RFC 2196 Site Security Handbook September 1997

1.1 Purpose of This Work

 This handbook is a guide to setting computer security policies and
 procedures for sites that have systems on the Internet (however, the
 information provided should also be useful to sites not yet connected
 to the Internet).  This guide lists issues and factors that a site
 must consider when setting their own policies.  It makes a number of
 recommendations and provides discussions of relevant areas.
 This guide is only a framework for setting security policies and
 procedures.  In order to have an effective set of policies and
 procedures, a site will have to make many decisions, gain agreement,
 and then communicate and implement these policies.

1.2 Audience

 The audience for this document are system and network administrators,
 and decision makers (typically "middle management") at sites.  For
 brevity, we will use the term "administrator" throughout this
 document to refer to system and network administrators.
 This document is not directed at programmers or those trying to
 create secure programs or systems.  The focus of this document is on
 the policies and procedures that need to be in place to support the
 technical security features that a site may be implementing.
 The primary audience for this work are sites that are members of the
 Internet community.  However, this document should be useful to any
 site that allows communication with other sites.  As a general guide
 to security policies, this document may also be useful to sites with
 isolated systems.

1.3 Definitions

 For the purposes of this guide, a "site" is any organization that
 owns computers or network-related resources. These resources may
 include host computers that users use, routers, terminal servers, PCs
 or other devices that have access to the Internet.  A site may be an
 end user of Internet services or a service provider such as a mid-
 level network.  However, most of the focus of this guide is on those
 end users of Internet services.  We assume that the site has the
 ability to set policies and procedures for itself with the
 concurrence and support from those who actually own the resources. It
 will be assumed that sites that are parts of larger organizations
 will know when they need to consult, collaborate, or take
 recommendations from, the larger entity.

Fraser, Ed. Informational [Page 3] RFC 2196 Site Security Handbook September 1997

 The "Internet" is a collection of thousands of networks linked by a
 common set of technical protocols which make it possible for users of
 any one of the networks to communicate with, or use the services
 located on, any of the other networks (FYI4, RFC 1594).
 The term "administrator" is used to cover all those people who are
 responsible for the day-to-day operation of system and network
 resources.  This may be a number of individuals or an organization.
 The term "security administrator" is used to cover all those people
 who are responsible for the security of information and information
 technology.  At some sites this function may be combined with
 administrator (above); at others, this will be a separate position.
 The term "decision maker" refers to those people at a site who set or
 approve policy.  These are often (but not always) the people who own
 the resources.

1.4 Related Work

 The Site Security Handbook Working Group is working on a User's Guide
 to Internet Security. It will provide practical guidance to end users
 to help them protect their information and the resources they use.

1.5 Basic Approach

 This guide is written to provide basic guidance in developing a
 security plan for your site.  One generally accepted approach to
 follow is suggested by Fites, et. al. [Fites 1989] and includes the
 following steps:
 (1)  Identify what you are trying to protect.
 (2)  Determine what you are trying to protect it from.
 (3)  Determine how likely the threats are.
 (4)  Implement measures which will protect your assets in a cost-
      effective manner.
 (5)  Review the process continuously and make improvements each time
      a weakness is found.
 Most of this document is focused on item 4 above, but the other steps
 cannot be avoided if an effective plan is to be established at your
 site.  One old truism in security is that the cost of protecting
 yourself against a threat should be less than the cost of recovering
 if the threat were to strike you.  Cost in this context should be
 remembered to include losses expressed in real currency, reputation,
 trustworthiness, and other less obvious measures.  Without reasonable
 knowledge of what you are protecting and what the likely threats are,
 following this rule could be difficult.

Fraser, Ed. Informational [Page 4] RFC 2196 Site Security Handbook September 1997

1.6 Risk Assessment

1.6.1 General Discussion

 One of the most important reasons for creating a computer security
 policy is to ensure that efforts spent on security yield cost
 effective benefits.  Although this may seem obvious, it is possible
 to be mislead about where the effort is needed.  As an example, there
 is a great deal of publicity about intruders on computers systems;
 yet most surveys of computer security show that, for most
 organizations, the actual loss from "insiders" is much greater.
 Risk analysis involves determining what you need to protect, what you
 need to protect it from, and how to protect it.  It is the process of
 examining all of your risks, then ranking those risks by level of
 severity.  This process involves making cost-effective decisions on
 what you want to protect.  As mentioned above, you should probably
 not spend more to protect something than it is actually worth.
 A full treatment of risk analysis is outside the scope of this
 document.  [Fites 1989] and [Pfleeger 1989] provide introductions to
 this topic.  However, there are two elements of a risk analysis that
 will be briefly covered in the next two sections:
 (1) Identifying the assets
 (2) Identifying the threats
 For each asset, the basic goals of security are availability,
 confidentiality, and integrity.  Each threat should be examined with
 an eye to how the threat could affect these areas.

1.6.2 Identifying the Assets

 One step in a risk analysis is to identify all the things that need
 to be protected.  Some things are obvious, like valuable proprietary
 information, intellectual property, and all the various pieces of
 hardware; but, some are overlooked, such as the people who actually
 use the systems. The essential point is to list all things that could
 be affected by a security problem.
 One list of categories is suggested by Pfleeger [Pfleeger 1989]; this
 list is adapted from that source:
 (1)  Hardware: CPUs, boards, keyboards, terminals,
      workstations, personal computers, printers, disk
      drives, communication lines, terminal servers, routers.

Fraser, Ed. Informational [Page 5] RFC 2196 Site Security Handbook September 1997

 (2)  Software: source programs, object programs,
      utilities, diagnostic programs, operating systems,
      communication programs.
 (3)  Data: during execution, stored on-line, archived off-line,
      backups, audit logs, databases, in transit over
      communication media.
 (4)  People: users, administrators, hardware maintainers.
 (5)  Documentation: on programs, hardware, systems, local
      administrative procedures.
 (6)  Supplies: paper, forms, ribbons, magnetic media.

1.6.3 Identifying the Threats

 Once the assets requiring protection are identified, it is necessary
 to identify threats to those assets.  The threats can then be
 examined to determine what potential for loss exists.  It helps to
 consider from what threats you are trying to protect your assets.
 The following are classic threats that should be considered.
 Depending on your site, there will be more specific threats that
 should be identified and addressed.
 (1)  Unauthorized access to resources and/or information
 (2)  Unintented and/or unauthorized Disclosure of information
 (3)  Denial of service

2. Security Policies

 Throughout this document there will be many references to policies.
 Often these references will include recommendations for specific
 policies. Rather than repeat guidance in how to create and
 communicate such a policy, the reader should apply the advice
 presented in this chapter when developing any policy recommended
 later in this book.

2.1 What is a Security Policy and Why Have One?

 The security-related decisions you make, or fail to make, as
 administrator largely determines how secure or insecure your network
 is, how much functionality your network offers, and how easy your
 network is to use.  However, you cannot make good decisions about
 security without first determining what your security goals are.
 Until you determine what your security goals are, you cannot make
 effective use of any collection of security tools because you simply
 will not know what to check for and what restrictions to impose.

Fraser, Ed. Informational [Page 6] RFC 2196 Site Security Handbook September 1997

 For example, your goals will probably be very different from the
 goals of a product vendor.  Vendors are trying to make configuration
 and operation of their products as simple as possible, which implies
 that the default configurations will often be as open (i.e.,
 insecure) as possible.  While this does make it easier to install new
 products, it also leaves access to those systems, and other systems
 through them, open to any user who wanders by.
 Your goals will be largely determined by the following key tradeoffs:
 (1)  services offered versus security provided -
      Each service offered to users carries its own security risks.
      For some services the risk outweighs the benefit of the service
      and the administrator may choose to eliminate the service rather
      than try to secure it.
 (2)  ease of use versus security -
      The easiest system to use would allow access to any user and
      require no passwords; that is, there would be no security.
      Requiring passwords makes the system a little less convenient,
      but more secure.  Requiring device-generated one-time passwords
      makes the system even more difficult to use, but much more
 (3)  cost of security versus risk of loss -
      There are many different costs to security: monetary (i.e., the
      cost of purchasing security hardware and software like firewalls
      and one-time password generators), performance (i.e., encryption
      and decryption take time), and ease of use (as mentioned above).
      There are also many levels of risk: loss of privacy (i.e., the
      reading of information by unauthorized individuals), loss of
      data (i.e., the corruption or erasure of information), and the
      loss of service (e.g., the filling of data storage space, usage
      of computational resources, and denial of network access).  Each
      type of cost must be weighed against each type of loss.
 Your goals should be communicated to all users, operations staff, and
 managers through a set of security rules, called a "security policy."
 We are using this term, rather than the narrower "computer security
 policy" since the scope includes all types of information technology
 and the information stored and manipulated by the technology.

2.1.1 Definition of a Security Policy

 A security policy is a formal statement of the rules by which people
 who are given access to an organization's technology and information
 assets must abide.

Fraser, Ed. Informational [Page 7] RFC 2196 Site Security Handbook September 1997

2.1.2 Purposes of a Security Policy

 The main purpose of a security policy is to inform users, staff and
 managers of their obligatory requirements for protecting technology
 and information assets.  The policy should specify the mechanisms
 through which these requirements can be met.  Another purpose is to
 provide a baseline from which to acquire, configure and audit
 computer systems and networks for compliance with the policy.
 Therefore an attempt to use a set of security tools in the absence of
 at least an implied security policy is meaningless.
 An Appropriate Use Policy (AUP) may also be part of a security
 policy.  It should spell out what users shall and shall not do on the
 various components of the system, including the type of traffic
 allowed on the networks.  The AUP should be as explicit as possible
 to avoid ambiguity or misunderstanding.  For example, an AUP might
 list any prohibited USENET newsgroups. (Note: Appropriate Use Policy
 is referred to as Acceptable Use Policy by some sites.)

2.1.3 Who Should be Involved When Forming Policy?

 In order for a security policy to be appropriate and effective, it
 needs to have the acceptance and support of all levels of employees
 within the organization.  It is especially important that corporate
 management fully support the security policy process otherwise there
 is little chance that they will have the intended impact.  The
 following is a list of individuals who should be involved in the
 creation and review of security policy documents:
 (1)  site security administrator
 (2)  information technology technical staff (e.g., staff from
      computing center)
 (3)  administrators of large user groups within the organization
      (e.g., business divisions, computer science department within a
      university, etc.)
 (4)  security incident response team
 (5)  representatives of the user groups affected by the security
 (6)  responsible management
 (7)  legal counsel (if appropriate)
 The list above is representative of many organizations, but is not
 necessarily comprehensive.  The idea is to bring in representation
 from key stakeholders, management who have budget and policy
 authority, technical staff who know what can and cannot be supported,
 and legal counsel who know the legal ramifications of various policy

Fraser, Ed. Informational [Page 8] RFC 2196 Site Security Handbook September 1997

 choices.  In some organizations, it may be appropriate to include EDP
 audit personnel.  Involving this group is important if resulting
 policy statements are to reach the broadest possible acceptance.  It
 is also relevant to mention that the role of legal counsel will also
 vary from country to country.

2.2 What Makes a Good Security Policy?

 The characteristics of a good security policy are:
 (1)  It must be implementable through system administration
      procedures, publishing of acceptable use guidelines, or other
      appropriate methods.
 (2)  It must be enforcible with security tools, where appropriate,
      and with sanctions, where actual prevention is not technically
 (3)  It must clearly define the areas of responsibility for the
      users, administrators, and management.
 The components of a good security policy include:
 (1)  Computer Technology Purchasing Guidelines which specify
      required, or preferred, security features.  These should
      supplement existing purchasing policies and guidelines.
 (2)  A Privacy Policy which defines reasonable expectations of
      privacy regarding such issues as monitoring of electronic mail,
      logging of keystrokes, and access to users' files.
 (3)  An Access Policy which defines access rights and privileges to
      protect assets from loss or disclosure by specifying acceptable
      use guidelines for users, operations staff, and management.  It
      should provide guidelines for external connections, data
      communications, connecting devices to a network, and adding new
      software to systems.  It should also specify any required
      notification messages (e.g., connect messages should provide
      warnings about authorized usage and line monitoring, and not
      simply say "Welcome").
 (4)  An Accountability Policy which defines the responsibilities of
      users, operations staff, and management.  It should specify an
      audit capability, and provide incident handling guidelines
      (i.e., what to do and who to contact if a possible intrusion is

Fraser, Ed. Informational [Page 9] RFC 2196 Site Security Handbook September 1997

 (5)  An Authentication Policy which establishes trust through an
      effective password policy, and by setting guidelines for remote
      location authentication and the use of authentication devices
      (e.g., one-time passwords and the devices that generate them).
 (6)  An Availability statement which sets users' expectations for the
      availability of resources.  It should address redundancy and
      recovery issues, as well as specify operating hours and
      maintenance down-time periods.  It should also include contact
      information for reporting system and network failures.
 (7)  An Information Technology System & Network Maintenance Policy
      which describes how both internal and external maintenance
      people are allowed to handle and access technology. One
      important topic to be addressed here is whether remote
      maintenance is allowed and how such access is controlled.
      Another area for consideration here is outsourcing and how it is
 (8)  A Violations Reporting Policy that indicates which types of
      violations (e.g., privacy and security, internal and external)
      must be reported and to whom the reports are made.  A non-
      threatening atmosphere and the possibility of anonymous
      reporting will result in a greater probability that a violation
      will be reported if it is detected.
 (9)  Supporting Information which provides users, staff, and
      management with contact information for each type of policy
      violation; guidelines on how to handle outside queries about a
      security incident, or information which may be considered
      confidential or proprietary; and cross-references to security
      procedures and related information, such as company policies and
      governmental laws and regulations.
 There may be regulatory requirements that affect some aspects of your
 security policy (e.g., line monitoring).  The creators of the
 security policy should consider seeking legal assistance in the
 creation of the policy.  At a minimum, the policy should be reviewed
 by legal counsel.
 Once your security policy has been established it should be clearly
 communicated to users, staff, and management.  Having all personnel
 sign a statement indicating that they have read, understood, and
 agreed to abide by the policy is an important part of the process.
 Finally, your policy should be reviewed on a regular basis to see if
 it is successfully supporting your security needs.

Fraser, Ed. Informational [Page 10] RFC 2196 Site Security Handbook September 1997

2.3 Keeping the Policy Flexible

 In order for a security policy to be viable for the long term, it
 requires a lot of flexibility based upon an architectural security
 concept. A security policy should be (largely) independent from
 specific hardware and software situations (as specific systems tend
 to be replaced or moved overnight).  The mechanisms for updating the
 policy should be clearly spelled out.  This includes the process, the
 people involved, and the people who must sign-off on the changes.
 It is also important to recognize that there are exceptions to every
 rule.  Whenever possible, the policy should spell out what exceptions
 to the general policy exist.  For example, under what conditions is a
 system administrator allowed to go through a user's files.  Also,
 there may be some cases when multiple users will have access to the
 same userid.  For example, on systems with a "root" user, multiple
 system administrators may know the password and use the root account.
 Another consideration is called the "Garbage Truck Syndrome."  This
 refers to what would happen to a site if a key person was suddenly
 unavailable for his/her job function (e.g., was suddenly ill or left
 the company unexpectedly).  While the greatest security resides in
 the minimum dissemination of information, the risk of losing critical
 information increases when that information is not shared.  It is
 important to determine what the proper balance is for your site.

3. Architecture

3.1 Objectives

3.1.1 Completely Defined Security Plans

 All sites should define a comprehensive security plan.  This plan
 should be at a higher level than the specific policies discussed in
 chapter 2, and it should be crafted as a framework of broad
 guidelines into which specific policies will fit.
 It is important to have this framework in place so that individual
 policies can be consistent with the overall site security
 architecture.  For example, having a strong policy with regard to
 Internet access and having weak restrictions on modem usage is
 inconsistent with an overall philosophy of strong security
 restrictions on external access.
 A security plan should define: the list of network services that will
 be provided; which areas of the organization will provide the
 services; who will have access to those services; how access will be
 provided; who will administer those services; etc.

Fraser, Ed. Informational [Page 11] RFC 2196 Site Security Handbook September 1997

 The plan should also address how incident will be handled.  Chapter 5
 provides an in-depth discussion of this topic, but it is important
 for each site to define classes of incidents and corresponding
 responses.  For example, sites with firewalls should set a threshold
 on the number of attempts made to foil the firewall before triggering
 a response?  Escallation levels should be defined for both attacks
 and responses.  Sites without firewalls will have to determine if a
 single attempt to connect to a host constitutes an incident? What
 about a systematic scan of systems?
 For sites connected to the Internet, the rampant media magnification
 of Internet related security incidents can overshadow a (potentially)
 more serious internal security problem.  Likewise, companies who have
 never been connected to the Internet may have strong, well defined,
 internal policies but fail to adequately address an external
 connection policy.

3.1.2 Separation of Services

 There are many services which a site may wish to provide for its
 users, some of which may be external.  There are a variety of
 security reasons to attempt to isolate services onto dedicated host
 computers.  There are also performance reasons in most cases, but a
 detailed discussion is beyond to scope of this document.
 The services which a site may provide will, in most cases, have
 different levels of access needs and models of trust.  Services which
 are essential to the security or smooth operation of a site would be
 better off being placed on a dedicated machine with very limited
 access (see Section 3.1.3 "deny all" model), rather than on a machine
 that provides a service (or services) which has traditionally been
 less secure, or requires greater accessability by users who may
 accidentally suborn security.
 It is also important to distinguish between hosts which operate
 within different models of trust (e.g., all the hosts inside of a
 firewall and any host on an exposed network).
 Some of the services which should be examined for potential
 separation are outlined in section 3.2.3. It is important to remember
 that security is only as strong as the weakest link in the chain.
 Several of the most publicized penetrations in recent years have been
 through the exploitation of vulnerabilities in electronic mail
 systems.  The intruders were not trying to steal electronic mail, but
 they used the vulnerability in that service to gain access to other

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 If possible, each service should be running on a different machine
 whose only duty is to provide a specific service.  This helps to
 isolate intruders and limit potential harm.

3.1.3 Deny all/ Allow all

 There are two diametrically opposed underlying philosophies which can
 be adopted when defining a security plan.  Both alternatives are
 legitimate models to adopt, and the choice between them will depend
 on the site and its needs for security.
 The first option is to turn off all services and then selectively
 enable services on a case by case basis as they are needed. This can
 be done at the host or network level as appropriate.  This model,
 which will here after be referred to as the "deny all" model, is
 generally more secure than the other model described in the next
 paragraph.  More work is required to successfully implement a "deny
 all" configuration as well as a better understanding of services.
 Allowing only known services provides for a better analysis of a
 particular service/protocol and the design of a security mechanism
 suited to the security level of the site.
 The other model, which will here after be referred to as the "allow
 all" model, is much easier to implement, but is generally less secure
 than the "deny all" model.  Simply turn on all services, usually the
 default at the host level, and allow all protocols to travel across
 network boundaries, usually the default at the router level.  As
 security holes become apparent, they are restricted or patched at
 either the host or network level.
 Each of these models can be applied to different portions of the
 site, depending on functionality requirements, administrative
 control, site policy, etc.  For example, the policy may be to use the
 "allow all" model when setting up workstations for general use, but
 adopt a "deny all" model when setting up information servers, like an
 email hub.  Likewise, an "allow all" policy may be adopted for
 traffic between LAN's internal to the site, but a "deny all" policy
 can be adopted between the site and the Internet.
 Be careful when mixing philosophies as in the examples above.  Many
 sites adopt the theory of a hard "crunchy" shell and a soft "squishy"
 middle.  They are willing to pay the cost of security for their
 external traffic and require strong security measures, but are
 unwilling or unable to provide similar protections internally.  This
 works fine as long as the outer defenses are never breached and the
 internal users can be trusted.  Once the outer shell (firewall) is
 breached, subverting the internal network is trivial.

Fraser, Ed. Informational [Page 13] RFC 2196 Site Security Handbook September 1997

3.1.4 Identify Real Needs for Services

 There is a large variety of services which may be provided, both
 internally and on the Internet at large.  Managing security is, in
 many ways, managing access to services internal to the site and
 managing how internal users access information at remote sites.
 Services tend to rush like waves over the Internet.  Over the years
 many sites have established anonymous FTP servers, gopher servers,
 wais servers, WWW servers, etc. as they became popular, but not
 particularly needed, at all sites.  Evaluate all new services that
 are established with a skeptical attitude to determine if they are
 actually needed or just the current fad sweeping the Internet.
 Bear in mind that security complexity can grow exponentially with the
 number of services provided.  Filtering routers need to be modified
 to support the new protocols.  Some protocols are inherently
 difficult to filter safely (e.g., RPC and UDP services), thus
 providing more openings to the internal network.  Services provided
 on the same machine can interact in catastrophic ways.  For example,
 allowing anonymous FTP on the same machine as the WWW server may
 allow an intruder to place a file in the anonymous FTP area and cause
 the HTTP server to execute it.

3.2 Network and Service Configuration

3.2.1 Protecting the Infrastructure

 Many network administrators go to great lengths to protect the hosts
 on their networks.  Few administrators make any effort to protect the
 networks themselves.  There is some rationale to this.  For example,
 it is far easier to protect a host than a network.  Also, intruders
 are likely to be after data on the hosts; damaging the network would
 not serve their purposes.  That said, there are still reasons to
 protect the networks.  For example, an intruder might divert network
 traffic through an outside host in order to examine the data (i.e.,
 to search for passwords).  Also, infrastructure includes more than
 the networks and the routers which interconnect them.  Infrastructure
 also includes network management (e.g., SNMP), services (e.g., DNS,
 NFS, NTP, WWW), and security (i.e., user authentication and access
 The infrastructure also needs protection against human error.  When
 an administrator misconfigures a host, that host may offer degraded
 service.  This only affects users who require that host and, unless

Fraser, Ed. Informational [Page 14] RFC 2196 Site Security Handbook September 1997

 that host is a primary server, the number of affected users will
 therefore be limited.  However, if a router is misconfigured, all
 users who require the network will be affected.  Obviously, this is a
 far larger number of users than those depending on any one host.

3.2.2 Protecting the Network

 There are several problems to which networks are vulnerable.  The
 classic problem is a "denial of service" attack.  In this case, the
 network is brought to a state in which it can no longer carry
 legitimate users' data.  There are two common ways this can be done:
 by attacking the routers and by flooding the network with extraneous
 traffic.  Please note that the term "router" in this section is used
 as an example of a larger class of active network interconnection
 components that also includes components like firewalls, proxy-
 servers, etc.
 An attack on the router is designed to cause it to stop forwarding
 packets, or to forward them improperly.  The former case may be due
 to a misconfiguration, the injection of a spurious routing update, or
 a "flood attack" (i.e., the router is bombarded with unroutable
 packets, causing its performance to degrade).  A flood attack on a
 network is similar to a flood attack on a router, except that the
 flood packets are usually broadcast.  An ideal flood attack would be
 the injection of a single packet which exploits some known flaw in
 the network nodes and causes them to retransmit the packet, or
 generate error packets, each of which is picked up and repeated by
 another host.  A well chosen attack packet can even generate an
 exponential explosion of transmissions.
 Another classic problem is "spoofing."  In this case, spurious
 routing updates are sent to one or more routers causing them to
 misroute packets.  This differs from a denial of service attack only
 in the purpose behind the spurious route.  In denial of service, the
 object is to make the router unusable; a state which will be quickly
 detected by network users.  In spoofing, the spurious route will
 cause packets to be routed to a host from which an intruder may
 monitor the data in the packets.  These packets are then re-routed to
 their correct destinations.  However, the intruder may or may not
 have altered the contents of the packets.
 The solution to most of these problems is to protect the routing
 update packets sent by the routing protocols in use (e.g., RIP-2,
 OSPF).  There are three levels of protection: clear-text password,
 cryptographic checksum, and encryption.  Passwords offer only minimal
 protection against intruders who do not have direct access to the
 physical networks.  Passwords also offer some protection against
 misconfigured routers (i.e, routers which, out of the box, attempt to

Fraser, Ed. Informational [Page 15] RFC 2196 Site Security Handbook September 1997

 route packets).  The advantage of passwords is that they have a very
 low overhead, in both bandwidth and CPU consumption.  Checksums
 protect against the injection of spurious packets, even if the
 intruder has direct access to the physical network.  Combined with a
 sequence number, or other unique identifier, a checksum can also
 protect again "replay" attacks, wherein an old (but valid at the
 time) routing update is retransmitted by either an intruder or a
 misbehaving router.  The most security is provided by complete
 encryption of sequenced, or uniquely identified, routing updates.
 This prevents an intruder from determining the topology of the
 network.  The disadvantage to encryption is the overhead involved in
 processing the updates.
 RIP-2 (RFC 1723) and OSPF (RFC 1583) both support clear-text
 passwords in their base design specifications.  In addition, there
 are extensions to each base protocol to support MD5 encryption.
 Unfortunately, there is no adequate protection against a flooding
 attack, or a misbehaving host or router which is flooding the
 network.  Fortunately, this type of attack is obvious when it occurs
 and can usually be terminated relatively simply.

3.2.3 Protecting the Services

 There are many types of services and each has its own security
 requirements.  These requirements will vary based on the intended use
 of the service.  For example, a service which should only be usable
 within a site (e.g., NFS) may require different protection mechanisms
 than a service provided for external use. It may be sufficient to
 protect the internal server from external access.  However, a WWW
 server, which provides a home page intended for viewing by users
 anywhere on the Internet, requires built-in protection.  That is, the
 service/protocol/server must provide whatever security may be
 required to prevent unauthorized access and modification of the Web
 Internal services (i.e., services meant to be used only by users
 within a site) and external services (i.e., services deliberately
 made available to users outside a site) will, in general, have
 protection requirements which differ as previously described.  It is
 therefore wise to isolate the internal services to one set of server
 host computers and the external services to another set of server
 host computers.  That is, internal and external servers should not be
 co-located on the same host computer.  In fact, many sites go so far

Fraser, Ed. Informational [Page 16] RFC 2196 Site Security Handbook September 1997

 as to have one set of subnets (or even different networks) which are
 accessible from the outside and another set which may be accessed
 only within the site.  Of course, there is usually a firewall which
 connects these partitions.  Great care must be taken to ensure that
 such a firewall is operating properly.
 There is increasing interest in using intranets to connect different
 parts of a organization (e.g., divisions of a company). While this
 document generally differentiates between external and internal
 (public and private), sites using intranets should be aware that they
 will need to consider three separations and take appropriate actions
 when designing and offering services. A service offered to an
 intranet would be neither public, nor as completely private as a
 service to a single organizational subunit. Therefore, the service
 would need its own supporting system, separated from both external
 and internal services and networks.
 One form of external service deserves some special consideration, and
 that is anonymous, or guest, access.  This may be either anonymous
 FTP or guest (unauthenticated) login.  It is extremely important to
 ensure that anonymous FTP servers and guest login userids are
 carefully isolated from any hosts and file systems from which outside
 users should be kept.  Another area to which special attention must
 be paid concerns anonymous, writable access.  A site may be legally
 responsible for the content of publicly available information, so
 careful monitoring of the information deposited by anonymous users is
 Now we shall consider some of the most popular services: name
 service, password/key service, authentication/proxy service,
 electronic mail, WWW, file transfer, and NFS.  Since these are the
 most frequently used services, they are the most obvious points of
 attack.  Also, a successful attack on one of these services can
 produce disaster all out of proportion to the innocence of the basic
 service. Name Servers (DNS and NIS(+))

 The Internet uses the Domain Name System (DNS) to perform address
 resolution for host and network names.  The Network Information
 Service (NIS) and NIS+ are not used on the global Internet, but are
 subject to the same risks as a DNS server.  Name-to-address
 resolution is critical to the secure operation of any network.  An
 attacker who can successfully control or impersonate a DNS server can
 re-route traffic to subvert security protections.  For example,
 routine traffic can be diverted to a compromised system to be
 monitored; or, users can be tricked into providing authentication
 secrets.  An organization should create well known, protected sites

Fraser, Ed. Informational [Page 17] RFC 2196 Site Security Handbook September 1997

 to act as secondary name servers and protect their DNS masters from
 denial of service attacks using filtering routers.
 Traditionally, DNS has had no security capabilities. In particular,
 the information returned from a query could not be checked for
 modification or verified that it had come from the name server in
 question.  Work has been done to incorporate digital signatures into
 the protocol which, when deployed, will allow the integrity of the
 information to be cryptographically verified (see RFC 2065). Password/Key Servers (NIS(+) and KDC)

 Password and key servers generally protect their vital information
 (i.e., the passwords and keys) with encryption algorithms.  However,
 even a one-way encrypted password can be determined by a dictionary
 attack (wherein common words are encrypted to see if they match the
 stored encryption).  It is therefore necessary to ensure that these
 servers are not accessable by hosts which do not plan to use them for
 the service, and even those hosts should only be able to access the
 service (i.e., general services, such as Telnet and FTP, should not
 be allowed by anyone other than administrators). Authentication/Proxy Servers (SOCKS, FWTK)

 A proxy server provides a number of security enhancements.  It allows
 sites to concentrate services through a specific host to allow
 monitoring, hiding of internal structure, etc.  This funnelling of
 services creates an attractive target for a potential intruder.  The
 type of protection required for a proxy server depends greatly on the
 proxy protocol in use and the services being proxied.  The general
 rule of limiting access only to those hosts which need the services,
 and limiting access by those hosts to only those services, is a good
 starting point. Electronic Mail

 Electronic mail (email) systems have long been a source for intruder
 break-ins because email protocols are among the oldest and most
 widely deployed services.  Also, by it's very nature, an email server
 requires access to the outside world; most email servers accept input
 from any source.  An email server generally consists of two parts: a
 receiving/sending agent and a processing agent.  Since email is
 delivered to all users, and is usually private, the processing agent
 typically requires system (root) privileges to deliver the mail.
 Most email implementations perform both portions of the service,
 which means the receiving agent also has system privileges.  This
 opens several security holes which this document will not describe.
 There are some implementations available which allow a separation of

Fraser, Ed. Informational [Page 18] RFC 2196 Site Security Handbook September 1997

 the two agents.  Such implementations are generally considered more
 secure, but still require careful installation to avoid creating a
 security problem. World Wide Web (WWW)

 The Web is growing in popularity exponentially because of its ease of
 use and the powerful ability to concentrate information services.
 Most WWW servers accept some type of direction and action from the
 persons accessing their services.  The most common example is taking
 a request from a remote user and passing the provided information to
 a program running on the server to process the request.  Some of
 these programs are not written with security in mind and can create
 security holes.  If a Web server is available to the Internet
 community, it is especially important that confidential information
 not be co-located on the same host as that server.  In fact, it is
 recommended that the server have a dedicated host which is not
 "trusted" by other internal hosts.
 Many sites may want to co-locate FTP service with their WWW service.
 But this should only occur for anon-ftp servers that only provide
 information (ftp-get). Anon-ftp puts, in combination with WWW, might
 be dangerous (e.g., they could result in modifications to the
 information your site is publishing to the web) and in themselves
 make the security considerations for each service different. File Transfer (FTP, TFTP)

 FTP and TFTP both allow users to receive and send electronic files in
 a point-to-point manner.  However, FTP requires authentication while
 TFTP requires none. For this reason, TFTP should be avoided as much
 as possible.
 Improperly configured FTP servers can allow intruders to copy,
 replace and delete files at will, anywhere on a host, so it is very
 important to configure this service correctly.   Access to encrypted
 passwords and proprietary data, and the introduction of Trojan horses
 are just a few of the potential security holes that can occur when
 the service is configured incorrectly. FTP servers should reside on
 their own host.  Some sites choose to co-locate FTP with a Web
 server, since the two protocols share common security considerations
 However, the the practice isn't recommended, especially when the FTP
 service allows the deposit of files (see section on WWW above). As
 mentioned in the opening paragraphs of section 3.2.3, services
 offered internally to your site should not be co-located with
 services offered externally.  Each should have its own host.

Fraser, Ed. Informational [Page 19] RFC 2196 Site Security Handbook September 1997

 TFTP does not support the same range of functions as FTP, and has no
 security whatsoever.  This service should only be considered for
 internal use, and then it should be configured in a restricted way so
 that the server only has access to a set of predetermined files
 (instead of every world-readable file on the system).  Probably the
 most common usage of TFTP is for downloading router configuration
 files to a router.  TFTP should reside on its own host, and should
 not be installed on hosts supporting external FTP or Web access. NFS

 The Network File Service allows hosts to share common disks.  NFS is
 frequently used by diskless hosts who depend on a disk server for all
 of their storage needs.  Unfortunately, NFS has no built-in security.
 It is therefore necessary that the NFS server be accessable only by
 those hosts which are using it for service.  This is achieved by
 specifying which hosts the file system is being exported to and in
 what manner (e.g., read-only, read-write, etc.). Filesystems should
 not be exported to any hosts outside the local network since this
 will require that the NFS service be accessible externally. Ideally,
 external access to NFS service should be stopped by a firewall.

3.2.4 Protecting the Protection

 It is amazing how often a site will overlook the most obvious
 weakness in its security by leaving the security server itself open
 to attack.  Based on considerations previously discussed, it should
 be clear that: the security server should not be accessible from
 off-site; should offer minimum access, except for the authentication
 function, to users on-site; and should not be co-located with any
 other servers.  Further, all access to the node, including access to
 the service itself, should be logged to provide a "paper trail" in
 the event of a security breach.

3.3 Firewalls

 One of the most widely deployed and publicized security measures in
 use on the Internet is a "firewall."  Firewalls have been given the
 reputation of a general panacea for many, if not all, of the Internet
 security issues.  They are not.  Firewalls are just another tool in
 the quest for system security.  They provide a certain level of
 protection and are, in general, a way of implementing security policy
 at the network level.  The level of security that a firewall provides
 can vary as much as the level of security on a particular machine.
 There are the traditional trade-offs between security, ease of use,
 cost, complexity, etc.

Fraser, Ed. Informational [Page 20] RFC 2196 Site Security Handbook September 1997

 A firewall is any one of several mechanisms used to control and watch
 access to and from a network for the purpose of protecting it.  A
 firewall acts as a gateway through which all traffic to and from the
 protected network and/or systems passes.  Firewalls help to place
 limitations on the amount and type of communication that takes place
 between the protected network and the another network (e.g., the
 Internet, or another piece of the site's network).
 A firewall is generally a way to build a wall between one part of a
 network, a company's internal network, for example, and another part,
 the global Internet, for example.  The unique feature about this wall
 is that there needs to be ways for some traffic with particular
 characteristics to pass through carefully monitored doors
 ("gateways").  The difficult part is establishing the criteria by
 which the packets are allowed or denied access through the doors.
 Books written on firewalls use different terminology to describe the
 various forms of firewalls. This can be confusing to system
 administrators who are not familiar with firewalls. The thing to note
 here is that there is no fixed terminology for the description of
 Firewalls are not always, or even typically, a single machine.
 Rather, firewalls are often a combination of routers, network
 segments, and host computers.  Therefore, for the purposes of this
 discussion, the term "firewall" can consist of more than one physical
 device.  Firewalls are typically built using two different
 components, filtering routers and proxy servers.
 Filtering routers are the easiest component to conceptualize in a
 firewall.  A router moves data back and forth between two (or more)
 different networks.  A "normal" router takes a packet from network A
 and "routes" it to its destination on network B.  A filtering router
 does the same thing but decides not only how to route the packet, but
 whether it should route the packet.  This is done by installing a
 series of filters by which the router decides what to do with any
 given packet of data.
 A discussion concerning capabilities of a particular brand of router,
 running a particular software version is outside the scope of this
 document.  However, when evaluating a router to be used for filtering
 packets, the following criteria can be important when implementing a
 filtering policy:  source and destination IP address, source and
 destination TCP port numbers, state of the TCP "ack" bit, UDP source
 and destination port numbers, and direction of packet flow (i.e.. A-
 >B or B->A).  Other information necessary to construct a secure
 filtering scheme are whether the router reorders filter instructions
 (designed to optimize filters, this can sometimes change the meaning
 and cause unintended access), and whether it is possible to apply

Fraser, Ed. Informational [Page 21] RFC 2196 Site Security Handbook September 1997

 filters for inbound and outbound packets on each interface (if the
 router filters only outbound packets then the router is "outside" of
 its filters and may be more vulnerable to attack).  In addition to
 the router being vulnerable, this distinction between applying
 filters on inbound or outbound packets is especially relevant for
 routers with more than 2 interfaces.  Other important issues are the
 ability to create filters based on IP header options and the fragment
 state of a packet.  Building a good filter can be very difficult and
 requires a good understanding of the type of services (protocols)
 that will be filtered.
 For better security, the filters usually restrict access between the
 two connected nets to just one host, the bastion host.  It is only
 possible to access the other network via this bastion host.  As only
 this host, rather than a few hundred hosts, can get attacked, it is
 easier to maintain a certain level of security because only this host
 has to be protected very carefully.  To make resources available to
 legitimate users across this firewall, services have to be forwarded
 by the bastion host.  Some servers have forwarding built in (like
 DNS-servers or SMTP-servers), for other services (e.g., Telnet, FTP,
 etc.), proxy servers can be used to allow access to the resources
 across the firewall in a secure way.
 A proxy server is way to concentrate application services through a
 single machine.  There is typically a single machine (the bastion
 host) that acts as a proxy server for a variety of protocols (Telnet,
 SMTP, FTP, HTTP, etc.) but there can be individual host computers for
 each service.  Instead of connecting directly to an external server,
 the client connects to the proxy server which in turn initiates a
 connection to the requested external server.  Depending on the type
 of proxy server used, it is possible to configure internal clients to
 perform this redirection automatically, without knowledge to the
 user, others might require that the user connect directly to the
 proxy server and then initiate the connection through a specified
 There are significant security benefits which can be derived from
 using proxy servers.  It is possible to add access control lists to
 protocols, requiring users or systems to provide some level of
 authentication before access is granted.  Smarter proxy servers,
 sometimes called Application Layer Gateways (ALGs), can be written
 which understand specific protocols and can be configured to block
 only subsections of the protocol.  For example, an ALG for FTP can
 tell the difference between the "put" command and the "get" command;
 an organization may wish to allow users to "get" files from the
 Internet, but not be able to "put" internal files on a remote server.
 By contrast, a filtering router could either block all FTP access, or
 none, but not a subset.

Fraser, Ed. Informational [Page 22] RFC 2196 Site Security Handbook September 1997

 Proxy servers can also be configured to encrypt data streams based on
 a variety of parameters.  An organization might use this feature to
 allow encrypted connections between two locations whose sole access
 points are on the Internet.
 Firewalls are typically thought of as a way to keep intruders out,
 but they are also often used as a way to let legitimate users into a
 site.  There are many examples where a valid user might need to
 regularly access the "home" site while on travel to trade shows and
 conferences, etc.  Access to the Internet is often available but may
 be through an untrusted machine or network.  A correctly configured
 proxy server can allow the correct users into the site while still
 denying access to other users.
 The current best effort in firewall techniques is found using a
 combination of a pair of screening routers with one or more proxy
 servers on a network between the two routers.  This setup allows the
 external router to block off any attempts to use the underlying IP
 layer to break security (IP spoofing, source routing, packet
 fragments), while allowing the proxy server to handle potential
 security holes in the higher layer protocols.  The internal router's
 purpose is to block all traffic except to the proxy server.  If this
 setup is rigidly implemented, a high level of security can be
 Most firewalls provide logging which can be tuned to make security
 administration of the network more convenient.  Logging may be
 centralized and the system may be configured to send out alerts for
 abnormal conditions.  It is important to regularly monitor these logs
 for any signs of intrusions or break-in attempts.  Since some
 intruders will attempt to cover their tracks by editing logs, it is
 desirable to protect these logs.  A variety of methods is available,
 including: write once, read many (WORM) drives; papers logs; and
 centralized logging via the "syslog" utility.  Another technique is
 to use a "fake" serial printer, but have the serial port connected to
 an isolated machine or PC which keeps the logs.
 Firewalls are available in a wide range of quality and strengths.
 Commercial packages start at approximately $10,000US and go up to
 over $250,000US.  "Home grown" firewalls can be built for smaller
 amounts of capital.  It should be remembered that the correct setup
 of a firewall (commercial or homegrown) requires a significant amount
 of skill and knowledge of TCP/IP.  Both types require regular
 maintenance, installation of software patches and updates, and
 regular monitoring.  When budgeting for a firewall, these additional
 costs should be considered in addition to the cost of the physical
 elements of the firewall.

Fraser, Ed. Informational [Page 23] RFC 2196 Site Security Handbook September 1997

 As an aside, building a "home grown" firewall requires a significant
 amount of skill and knowledge of TCP/IP.  It should not be trivially
 attempted because a perceived sense of security is worse in the long
 run than knowing that there is no security.  As with all security
 measures, it is important to decide on the threat, the value of the
 assets to be protected, and the costs to implement security.
 A final note about firewalls.  They can be a great aid when
 implementing security for a site and they protect against a large
 variety of attacks.  But it is important to keep in mind that they
 are only one part of the solution.  They cannot protect your site
 against all types of attack.

4. Security Services and Procedures

 This chapter guides the reader through a number of topics that should
 be addressed when securing a site.  Each section touches on a
 security service or capability that may be required to protect the
 information and systems at a site.  The topics are presented at a
 fairly high-level to introduce the reader to the concepts.
 Throughout the chapter, you will find significant mention of
 cryptography.  It is outside the scope of this document to delve into
 details concerning cryptography, but the interested reader can obtain
 more information from books and articles listed in the reference
 section of this document.

4.1 Authentication

 For many years, the prescribed method for authenticating users has
 been through the use of standard, reusable passwords.  Originally,
 these passwords were used by users at terminals to authenticate
 themselves to a central computer.  At the time, there were no
 networks (internally or externally), so the risk of disclosure of the
 clear text password was minimal.  Today, systems are connected
 together through local networks, and these local networks are further
 connected together and to the Internet.  Users are logging in from
 all over the globe; their reusable passwords are often transmitted
 across those same networks in clear text, ripe for anyone in-between
 to capture.  And indeed, the CERT* Coordination Center and other
 response teams are seeing a tremendous number of incidents involving
 packet sniffers which are capturing the clear text passwords.
 With the advent of newer technologies like one-time passwords (e.g.,
 S/Key), PGP, and token-based authentication devices, people are using
 password-like strings as secret tokens and pins.  If these secret
 tokens and pins are not properly selected and protected, the
 authentication will be easily subverted.

Fraser, Ed. Informational [Page 24] RFC 2196 Site Security Handbook September 1997

4.1.1 One-Time passwords

 As mentioned above, given today's networked environments, it is
 recommended that sites concerned about the security and integrity of
 their systems and networks consider moving away from standard,
 reusable passwords.  There have been many incidents involving Trojan
 network programs (e.g., telnet and rlogin) and network packet
 sniffing programs.  These programs capture clear text
 hostname/account name/password triplets.  Intruders can use the
 captured information for subsequent access to those hosts and
 accounts.  This is possible because 1) the password is used over and
 over (hence the term "reusable"), and 2) the password passes across
 the network in clear text.
 Several authentication techniques have been developed that address
 this problem.  Among these techniques are challenge-response
 technologies that provide passwords that are only used once (commonly
 called one-time passwords). There are a number of products available
 that sites should consider using. The decision to use a product is
 the responsibility of each organization, and each organization should
 perform its own evaluation and selection.

4.1.2 Kerberos

 Kerberos is a distributed network security system which provides for
 authentication across unsecured networks.  If requested by the
 application, integrity and encryption can also be provided.  Kerberos
 was originally developed at the Massachusetts Institute of Technology
 (MIT) in the mid 1980s.  There are two major releases of Kerberos,
 version 4 and 5, which are for practical purposes, incompatible.
 Kerberos relies on a symmetric key database using a key distribution
 center (KDC) which is known as the Kerberos server.  A user or
 service (known as "principals") are granted electronic "tickets"
 after properly communicating with the KDC.  These tickets are used
 for authentication between principals.  All tickets include a time
 stamp which limits the time period for which the ticket is valid.
 Therefore, Kerberos clients and server must have a secure time
 source, and be able to keep time accurately.
 The practical side of Kerberos is its integration with the
 application level.  Typical applications like FTP, telnet, POP, and
 NFS have been integrated with the Kerberos system.  There are a
 variety of implementations which have varying levels of integration.
 Please see the Kerberos FAQ available at
 faq.html for the latest information.

Fraser, Ed. Informational [Page 25] RFC 2196 Site Security Handbook September 1997

4.1.3 Choosing and Protecting Secret Tokens and PINs

 When selecting secret tokens, take care to choose them carefully.
 Like the selection of passwords, they should be robust against brute
 force efforts to guess them.  That is, they should not be single
 words in any language, any common, industry, or cultural acronyms,
 etc.  Ideally, they will be longer rather than shorter and consist of
 pass phrases that combine upper and lower case character, digits, and
 other characters.
 Once chosen, the protection of these secret tokens is very important.
 Some are used as pins to hardware devices (like token cards) and
 these should not be written down or placed in the same location as
 the device with which they are associated.  Others, such as a secret
 Pretty Good Privacy (PGP) key, should be protected from unauthorized
 One final word on this subject.  When using cryptography products,
 like PGP, take care to determine the proper key length and ensure
 that your users are trained to do likewise.  As technology advances,
 the minimum safe key length continues to grow.  Make sure your site
 keeps up with the latest knowledge on the technology so that you can
 ensure that any cryptography in use is providing the protection you
 believe it is.

4.1.4 Password Assurance

 While the need to eliminate the use of standard, reusable passwords
 cannot be overstated, it is  recognized that some organizations may
 still be using them.  While it's recommended that these organizations
 transition to the use of better technology, in the mean time, we have
 the following advice to help with the selection and maintenance of
 traditional passwords. But remember, none of these measures provides
 protection against disclosure due to sniffer programs.
 (1)  The importance of robust passwords - In many (if not most) cases
      of system penetration, the intruder needs to gain access to an
      account on the system. One way that goal is typically
      accomplished is through guessing the password of a legitimate
      user.  This is often accomplished by running an automated
      password cracking program, which utilizes a very large
      dictionary, against the system's password file.  The only way to
      guard against passwords being disclosed in this manner is
      through the careful selection of passwords which cannot be
      easily guessed (i.e., combinations of numbers, letters, and
      punctuation characters).  Passwords should also be as long as
      the system supports and users can tolerate.

Fraser, Ed. Informational [Page 26] RFC 2196 Site Security Handbook September 1997

 (2)  Changing default passwords - Many operating systems and
      application programs are installed with default accounts and
      passwords.  These must be changed immediately to something that
      cannot be guessed or cracked.
 (3)  Restricting access to the password file - In particular, a site
      wants to protect the encrypted password portion of the file so
      that would-be intruders don't have them available for cracking.
      One effective technique is to use shadow passwords where the
      password field of the standard file contains a dummy or false
      password.  The file containing the legitimate passwords are
      protected elsewhere on the system.
 (4)  Password aging - When and how to expire passwords is still a
      subject of controversy among the security community.  It is
      generally accepted that a password should not be maintained once
      an account is no longer in use, but it is hotly debated whether
      a user should be forced to change a good password that's in
      active use.  The arguments for changing passwords relate to the
      prevention of the continued use of penetrated accounts.
      However, the opposition claims that frequent password changes
      lead to users writing down their passwords in visible areas
      (such as pasting them to a terminal), or to users selecting very
      simple passwords that are easy to guess.  It should also be
      stated that an intruder will probably use a captured or guessed
      password sooner rather than later, in which case password aging
      provides little if any protection.
      While there is no definitive answer to this dilemma, a password
      policy should directly address the issue and provide guidelines
      for how often a user should change the password.  Certainly, an
      annual change in their password is usually not difficult for
      most users, and you should consider requiring it.  It is
      recommended that passwords be changed at least whenever a
      privileged account is compromised, there is a critical change in
      personnel (especially if it is an administrator!), or when an
      account has been compromised.  In addition, if a privileged
      account password is compromised, all passwords on the system
      should be changed.
 (5)  Password/account blocking - Some sites find it useful to disable
      accounts after a predefined number of failed attempts to
      authenticate.  If your site decides to employ this mechanism, it
      is recommended that the mechanism not "advertise" itself. After

Fraser, Ed. Informational [Page 27] RFC 2196 Site Security Handbook September 1997

      disabling, even if the correct password is presented, the
      message displayed should remain that of a failed login attempt.
      Implementing this mechanism will require that legitimate users
      contact their system administrator to request that their account
      be reactivated.
 (6)  A word about the finger daemon - By default, the finger daemon
      displays considerable system and user information. For example,
      it can display a list of all users currently using a system, or
      all the contents of a specific user's .plan file.  This
      information can be used by would-be intruders to identify
      usernames and guess their passwords. It is recommended that
      sites consider modifying finger to restrict the information

4.2 Confidentiality

 There will be information assets that your site will want to protect
 from disclosure to unauthorized entities.  Operating systems often
 have built-in file protection mechanisms that allow an administrator
 to control who on the system can access, or "see," the contents of a
 given file.  A stronger way to provide confidentiality is through
 encryption.  Encryption is accomplished by scrambling data so that it
 is very difficult and time consuming for anyone other than the
 authorized recipients or owners to obtain the plain text.  Authorized
 recipients and the owner of the information will possess the
 corresponding decryption keys that allow them to easily unscramble
 the text to a readable (clear text) form.  We recommend that sites
 use encryption to provide confidentiality and protect valuable
 The use of encryption is sometimes controlled by governmental and
 site regulations, so we encourage administrators to become informed
 of laws or policies that regulate its use before employing it.  It is
 outside the scope of this document to discuss the various algorithms
 and programs available for this purpose, but we do caution against
 the casual use of the UNIX crypt program as it has been found to be
 easily broken.  We also encourage everyone to take time to understand
 the strength of the encryption in any given algorithm/product before
 using it.  Most well-known products are well-documented in the
 literature, so this should be a fairly easy task.

4.3 Integrity

 As an administrator, you will want to make sure that information
 (e.g., operating system files, company data, etc.) has not been
 altered in an unauthorized fashion.  This means you will want to
 provide some assurance as to the integrity of the information on your

Fraser, Ed. Informational [Page 28] RFC 2196 Site Security Handbook September 1997

 systems.  One way to provide this is to produce a checksum of the
 unaltered file, store that checksum offline, and periodically (or
 when desired) check to make sure the checksum of the online file
 hasn't changed (which would indicate the data has been modified).
 Some operating systems come with checksumming programs, such as the
 UNIX sum program.  However, these may not provide the protection you
 actually need.  Files can be modified in such a way as to preserve
 the result of the UNIX sum program!  Therefore, we suggest that you
 use a cryptographically strong program, such as the message digesting
 program MD5 [ref], to produce the checksums you will be using to
 assure integrity.
 There are other applications where integrity will need to be assured,
 such as when transmitting an email message between two parties. There
 are products available that can provide this capability.  Once you
 identify that this is a capability you need, you can go about
 identifying technologies that will provide it.

4.4 Authorization

 Authorization refers to the process of granting privileges to
 processes and, ultimately, users.  This differs from authentication
 in that authentication is the process used to identify a user.  Once
 identified (reliably), the privileges, rights, property, and
 permissible actions of the user are determined by authorization.
 Explicitly listing the authorized activities of each user (and user
 process) with respect to all resources (objects) is impossible in a
 reasonable system.  In a real system certain techniques are used to
 simplify the process of granting and checking authorization(s).
 One approach, popularized in UNIX systems, is to assign to each
 object three classes of user: owner, group and world.  The owner is
 either the creator of the object or the user assigned as owner by the
 super-user.  The owner permissions (read, write and execute) apply
 only to the owner.  A group is a collection of users which share
 access rights to an object.  The group permissions (read, write and
 execute) apply to all users in the group (except the owner).  The
 world refers to everybody else with access to the system.  The world
 permissions (read, write and execute) apply to all users (except the
 owner and members of the group).
 Another approach is to attach to an object a list which explicitly
 contains the identity of all permitted users (or groups).  This is an
 Access Control List (ACL).  The advantage of ACLs are that they are

Fraser, Ed. Informational [Page 29] RFC 2196 Site Security Handbook September 1997

 easily maintained (one central list per object) and it's very easy to
 visually check who has access to what. The disadvantages are the
 extra resources required to store such lists, as well as the vast
 number of such lists required for large systems.

4.5 Access

4.5.1 Physical Access

 Restrict physical access to hosts, allowing access only to those
 people who are supposed to use the hosts.  Hosts include "trusted"
 terminals (i.e., terminals which allow unauthenticated use such as
 system consoles, operator terminals and terminals dedicated to
 special tasks), and individual microcomputers and workstations,
 especially those connected to your network.  Make sure people's work
 areas mesh well with access restrictions; otherwise they will find
 ways to circumvent your physical security (e.g., jamming doors open).
 Keep original and backup copies of data and programs safe.  Apart
 from keeping them in good condition for backup purposes, they must be
 protected from theft.  It is important to keep backups in a separate
 location from the originals, not only for damage considerations, but
 also to guard against thefts.
 Portable hosts are a particular risk.  Make sure it won't cause
 problems if one of your staff's portable computer is stolen.
 Consider developing guidelines for the kinds of data that should be
 allowed to reside on the disks of portable computers as well as how
 the data should be protected (e.g., encryption) when it is on a
 portable computer.
 Other areas where physical access should be restricted is the wiring
 closets and important network elements like file servers, name server
 hosts, and routers.

4.5.2 Walk-up Network Connections

 By "walk-up" connections, we mean network connection points located
 to provide a convenient way for users to connect a portable host to
 your network.
 Consider whether you need to provide this service, bearing in mind
 that it allows any user to attach an unauthorized host to your
 network.  This increases the risk of attacks via techniques such as

Fraser, Ed. Informational [Page 30] RFC 2196 Site Security Handbook September 1997

 IP address spoofing, packet sniffing, etc.  Users and site management
 must appreciate the risks involved.  If you decide to provide walk-up
 connections, plan the service carefully and define precisely where
 you will provide it so that you can ensure the necessary physical
 access security.
 A walk-up host should be authenticated before its user is permitted
 to access resources on your network.  As an alternative, it may be
 possible to control physical access. For example, if the service is
 to be used by students, you might only provide walk-up connection
 sockets in student laboratories.
 If you are providing walk-up access for visitors to connect back to
 their home networks (e.g., to read e-mail, etc.) in your facility,
 consider using a separate subnet that has no connectivity to the
 internal network.
 Keep an eye on any area that contains unmonitored access to the
 network, such as vacant offices.  It may be sensible to disconnect
 such areas at the wiring closet, and consider using secure hubs and
 monitoring attempts to connect unauthorized hosts.

4.5.3 Other Network Technologies

 Technologies considered here include X.25, ISDN, SMDS, DDS and Frame
 Relay.  All are provided via physical links which go through
 telephone exchanges, providing the potential for them to be diverted.
 Crackers are certainly interested in telephone switches as well as in
 data networks!
 With switched technologies, use Permanent Virtual Circuits or Closed
 User Groups whenever this is possible.  Technologies which provide
 authentication and/or encryption (such as IPv6) are evolving rapidly;
 consider using them on links where security is important.

4.5.4 Modems Modem Lines Must Be Managed

 Although they provide convenient access to a site for its users, they
 can also provide an effective detour around the site's firewalls.
 For this reason it is essential to maintain proper control of modems.
 Don't allow users to install a modem line without proper
 authorization.  This includes temporary installations (e.g., plugging
 a modem into a facsimile or telephone line overnight).

Fraser, Ed. Informational [Page 31] RFC 2196 Site Security Handbook September 1997

 Maintain a register of all your modem lines and keep your register up
 to date.  Conduct regular (ideally automated) site checks for
 unauthorized modems. Dial-in Users Must Be Authenticated

 A username and password check should be completed before a user can
 access anything on your network.  Normal password security
 considerations are particularly important (see section 4.1.1).
 Remember that telephone lines can be tapped, and that it is quite
 easy to intercept messages to cellular phones.  Modern high-speed
 modems use more sophisticated modulation techniques, which makes them
 somewhat more difficult to monitor, but it is prudent to assume that
 hackers know how to eavesdrop on your lines.  For this reason, you
 should use one-time passwords if at all possible.
 It is helpful to have a single dial-in point (e.g., a single large
 modem pool) so that all users are authenticated in the same way.
 Users will occasionally mis-type a password.  Set a short delay - say
 two seconds - after the first and second failed logins, and force a
 disconnect after the third.  This will slow down automated password
 attacks.  Don't tell the user whether the username, the password, or
 both, were incorrect. Call-back Capability

 Some dial-in servers offer call-back facilities (i.e., the user dials
 in and is authenticated, then the system disconnects the call and
 calls back on a specified number).  Call-back is useful since if
 someone were to guess a username and password, they are disconnected,
 and the system then calls back the actual user whose password was
 cracked; random calls from a server are suspicious, at best.  This
 does mean users may only log in from one location (where the server
 is configured to dial them back), and of course there may be phone
 charges associated with there call-back location.
 This feature should be used with caution; it can easily be bypassed.
 At a minimum, make sure that the return call is never made from the
 same modem as the incoming one.  Overall, although call-back can
 improve modem security, you should not depend on it alone. All Logins Should Be Logged

 All logins, whether successful or unsuccessful should be logged.
 However, do not keep correct passwords in the log. Rather, log them
 simply as a successful login attempt.  Since most bad passwords are

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 mistyped by authorized users, they only vary by a single character
 from the actual password.  Therefore if you can't keep such a log
 secure, don't log it at all.
 If Calling Line Identification is available, take advantage of it by
 recording the calling number for each login attempt.  Be sensitive to
 the privacy issues raised by Calling Line Identification.  Also be
 aware that Calling Line Identification is not to be trusted (since
 intruders have been known to break into phone switches and forward
 phone numbers or make other changes); use the data for informational
 purposes only, not for authentication. Choose Your Opening Banner Carefully

 Many sites use a system default contained in a message of the day
 file for their opening banner. Unfortunately, this often includes the
 type of host hardware or operating system present on the host.  This
 can provide valuable information to a would-be intruder. Instead,
 each site should create its own specific login banner, taking care to
 only include necessary information.
 Display a short banner, but don't offer an "inviting" name (e.g.,
 University of XYZ, Student Records System).  Instead, give your site
 name, a short warning that sessions may be monitored, and a
 username/password prompt.  Verify possible legal issues related to
 the text you put into the banner.
 For high-security applications, consider using a "blind" password
 (i.e., give no response to an incoming call until the user has typed
 in a password).  This effectively simulates a dead modem. Dial-out Authentication

 Dial-out users should also be authenticated, particularly since your
 site will have to pay their telephone charges.
 Never allow dial-out from an unauthenticated dial-in call, and
 consider whether you will allow it from an authenticated one.  The
 goal here is to prevent callers using your modem pool as part of a
 chain of logins.  This can be hard to detect, particularly if a
 hacker sets up a path through several hosts on your site.
 At a minimum, don't allow the same modems and phone lines to be used
 for both dial-in and dial-out.  This can be implemented easily if you
 run separate dial-in and dial-out modem pools.

Fraser, Ed. Informational [Page 33] RFC 2196 Site Security Handbook September 1997 Make Your Modem Programming as "Bullet-proof" as Possible

 Be sure modems can't be reprogrammed while they're in service.  At a
 minimum, make sure that three plus signs won't put your dial-in
 modems into command mode!
 Program your modems to reset to your standard configuration at the
 start of each new call.  Failing this, make them reset at the end of
 each call.  This precaution will protect you against accidental
 reprogramming of your modems. Resetting at both the end and the
 beginning of each call will assure an even higher level of confidence
 that a new caller will not inherit a previous caller's session.
 Check that your modems terminate calls cleanly.  When a user logs out
 from an access server, verify that the server hangs up the phone line
 properly.  It is equally important that the server forces logouts
 from whatever sessions were active if the user hangs up unexpectedly.

4.6 Auditing

 This section covers the procedures for collecting data generated by
 network activity, which may be useful in analyzing the security of a
 network and responding to security incidents.

4.6.1 What to Collect

 Audit data should include any attempt to achieve a different security
 level by any person, process, or other entity in the network.  This
 includes login and logout, super user access (or the non-UNIX
 equivalent), ticket generation (for Kerberos, for example), and any
 other change of access or status.  It is especially important to note
 "anonymous" or "guest" access to public servers.
 The actual data to collect will differ for different sites and for
 different types of access changes within a site.  In general, the
 information you want to collect includes: username and hostname, for
 login and logout; previous and new access rights, for a change of
 access rights; and a timestamp.  Of course, there is much more
 information which might be gathered, depending on what the system
 makes available and how much space is available to store that
 One very important note: do not gather passwords.  This creates an
 enormous potential security breach if the audit records should be
 improperly accessed.  Do not gather incorrect passwords either, as
 they often differ from valid passwords by only a single character or

Fraser, Ed. Informational [Page 34] RFC 2196 Site Security Handbook September 1997

4.6.2 Collection Process

 The collection process should be enacted by the host or resource
 being accessed.  Depending on the importance of the data and the need
 to have it local in instances in which services are being denied,
 data could be kept local to the resource until needed or be
 transmitted to storage after each event.
 There are basically three ways to store audit records: in a
 read/write file on a host, on a write-once/read-many device (e.g., a
 CD-ROM or a specially configured tape drive), or on a write-only
 device (e.g., a line printer).  Each method has advantages and
 File system logging is the least resource intensive of the three
 methods and the easiest to configure.  It allows instant access to
 the records for analysis, which may be important if an attack is in
 progress.  File system logging is also the least reliable method.  If
 the logging host has been compromised, the file system is usually the
 first thing to go; an intruder could easily cover up traces of the
 Collecting audit data on a write-once device is slightly more effort
 to configure than a simple file, but it has the significant advantage
 of greatly increased security because an intruder could not alter the
 data showing that an intrusion has occurred.  The disadvantage of
 this method is the need to maintain a supply of storage media and the
 cost of that media.  Also, the data may not be instantly available.
 Line printer logging is useful in system where permanent and
 immediate logs are required.  A real time system is an example of
 this, where the exact point of a failure or attack must be recorded.
 A laser printer, or other device which buffers data (e.g., a print
 server), may suffer from lost data if buffers contain the needed data
 at a critical instant.  The disadvantage of, literally, "paper
 trails" is the need to keep the printer fed and the need to scan
 records by hand.  There is also the issue of where to store the,
 potentially, enormous volume of paper which may be generated.
 For each of the logging methods described, there is also the issue of
 securing the path between the device generating the log and actual
 logging device (i.e., the file server, tape/CD-ROM drive, printer).
 If that path is compromised, logging can be stopped or spoofed or
 both.  In an ideal world, the logging device would be directly

Fraser, Ed. Informational [Page 35] RFC 2196 Site Security Handbook September 1997

 attached by a single, simple, point-to-point cable.  Since that is
 usually impractical, the path should pass through the minimum number
 of networks and routers.  Even if logs can be blocked, spoofing can
 be prevented with cryptographic checksums (it probably isn't
 necessary to encrypt the logs because they should not contain
 sensitive information in the first place).

4.6.3 Collection Load

 Collecting audit data may result in a rapid accumulation of bytes so
 storage availability for this information must be considered in
 advance.  There are a few ways to reduce the required storage space.
 First, data can be compressed, using one of many methods. Or, the
 required space can be minimized by keeping data for a shorter period
 of time with only summaries of that data kept in long-term archives.
 One major drawback to the latter method involves incident response.
 Often, an incident has been ongoing for some period of time when a
 site notices it and begins to investigate. At that point in time,
 it's very helpful to have detailed audit logs available. If these are
 just summaries, there may not be sufficient detail to fully handle
 the incident.

4.6.4 Handling and Preserving Audit Data

 Audit data should be some of the most carefully secured data at the
 site and in the backups.  If an intruder were to gain access to audit
 logs, the systems themselves, in addition to the data, would be at
 Audit data may also become key to the investigation, apprehension,
 and prosecution of the perpetrator of an incident.  For this reason,
 it is advisable to seek the advice of legal council when deciding how
 audit data should be treated.  This should happen before an incident
 If a data handling plan is not adequately defined prior to an
 incident, it may mean that there is no recourse in the aftermath of
 an event, and it may create liability resulting from improper
 treatment of the data.

4.6.5 Legal Considerations

 Due to the content of audit data, there are a number of legal
 questions that arise which might need to be addressed by your legal
 counsel. If you collect and save audit data, you need to be prepared
 for consequences resulting both from its existence and its content.

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 One area concerns the privacy of individuals.  In certain instances,
 audit data may contain personal information.  Searching through the
 data, even for a routine check of the system's security, could
 represent an invasion of privacy.
 A second area of concern involves knowledge of intrusive behavior
 originating from your site.  If an organization keeps audit data, is
 it responsible for examining it to search for incidents?  If a host
 in one organization is used as a launching point for an attack
 against another organization, can the second organization use the
 audit data of the first organization to prove negligence on the part
 of that organization?
 The above examples are meant to be comprehensive, but should motivate
 your organization to consider the legal issues involved with audit

4.7 Securing Backups

 The procedure of creating backups is a classic part of operating a
 computer system.  Within the context of this document, backups are
 addressed as part of the overall security plan of a site.  There are
 several aspects to backups that are important within this context:
 (1)  Make sure your site is creating backups
 (2)  Make sure your site is using offsite storage for backups. The
      storage site should be carefully selected for both its security
      and its availability.
 (3)  Consider encrypting your backups to provide additional protection
      of the information once it is off-site.  However, be aware that
      you will need a good key management scheme so that you'll be
      able to recover data at any point in the future.  Also, make
      sure you will have access to the necessary decryption programs
      at such time in the future as you need to perform the
 (4)  Don't always assume that your backups are good.  There have been
      many instances of computer security incidents that have gone on
      for long periods of time before a site has noticed the incident.
      In such cases, backups of the affected systems are also tainted.
 (5)  Periodically verify the correctness and completeness of your

5. Security Incident Handling

 This chapter of the document will supply guidance to be used before,
 during, and after a computer security incident occurs on a host,
 network, site, or multi-site environment.  The operative philosophy
 in the event of a breach of computer security is to react according

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 to a plan.  This is true whether the breach is the result of an
 external intruder attack, unintentional damage, a student testing
 some new program to exploit a software vulnerability, or a
 disgruntled employee.  Each of the possible types of events, such as
 those just listed, should be addressed in advance by adequate
 contingency plans.
 Traditional computer security, while quite important in the overall
 site security plan, usually pays little attention to how to actually
 handle an attack once one occurs.  The result is that when an attack
 is in progress, many decisions are made in haste and can be damaging
 to tracking down the source of the incident, collecting evidence to
 be used in prosecution efforts, preparing for the recovery of the
 system, and protecting the valuable data contained on the system.
 One of the most important, but often overlooked, benefits for
 efficient incident handling is an economic one.  Having both
 technical and managerial personnel respond to an incident requires
 considerable resources.  If trained to handle incidents efficiently,
 less staff time is required when one occurs.
 Due to the world-wide network most incidents are not restricted to a
 single site.  Operating systems vulnerabilities apply (in some cases)
 to several millions of systems, and many vulnerabilities are
 exploited within the network itself.  Therefore, it is vital that all
 sites with involved parties be informed as soon as possible.
 Another benefit is related to public relations.  News about computer
 security incidents tends to be damaging to an organization's stature
 among current or potential clients.  Efficient incident handling
 minimizes the potential for negative exposure.
 A final benefit of efficient incident handling is related to legal
 issues.  It is possible that in the near future organizations may be
 held responsible because one of their nodes was used to launch a
 network attack.   In a similar vein, people who develop patches or
 workarounds may be sued if the patches or workarounds are
 ineffective, resulting in compromise of the systems, or, if the
 patches or workarounds themselves damage systems.  Knowing about
 operating system vulnerabilities and patterns of attacks, and then
 taking appropriate measures to counter these potential threats, is
 critical to circumventing possible legal problems.

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 The sections in this chapter provide an outline and starting point
 for creating your site's policy for handling security incidents.  The
 sections are:
 (1)  Preparing and planning (what are the goals and objectives in
      handling an incident).
 (2)  Notification (who should be contacted in the case of an
        - Local managers and personnel
        - Law enforcement and investigative agencies
        - Computer security incidents handling teams
        - Affected and involved sites
        - Internal communications
        - Public relations and press releases
 (3)  Identifying an incident (is it an incident and how serious is
 (4)  Handling (what should be done when an incident occurs).
        - Notification (who should be notified about the incident)
        - Protecting evidence and activity logs (what records should be
          kept from before, during, and after the incident)
        - Containment (how can the damage be limited)
        - Eradication (how to eliminate the reasons for the incident)
        - Recovery (how to reestablish service and systems)
        - Follow Up (what actions should be taken after the incident)
 (5)  Aftermath (what are the implications of past incidents).
 (6)  Administrative response to incidents.
 The remainder of this chapter will detail the issues involved in each
 of the important topics listed above, and provide some guidance as to
 what should be included in a site policy for handling incidents.

5.1 Preparing and Planning for Incident Handling

 Part of handling an incident is being prepared to respond to an
 incident before the incident occurs in the first place.  This
 includes establishing a suitable level of protections as explained in
 the preceding chapters.  Doing this should help your site prevent
 incidents as well as limit potential damage resulting from them when
 they do occur.  Protection also includes preparing incident handling
 guidelines as part of a contingency plan for your organization or
 site.  Having written plans eliminates much of the ambiguity which
 occurs during an incident, and will lead to a more appropriate and
 thorough set of responses.  It is vitally important to test the
 proposed plan before an incident occurs through "dry runs".  A team
 might even consider hiring a tiger team to act in parallel with the
 dry run.  (Note: a tiger team is a team of specialists that try to
 penetrate the security of a system.)

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 Learning to respond efficiently to an incident is important for a
 number of reasons:
 (1)  Protecting the assets which could be compromised
 (2)  Protecting resources which could be utilized more
      profitably if an incident did not require their services
 (3)  Complying with (government or other) regulations
 (4)  Preventing the use of your systems in attacks against other
      systems (which could cause you to incur legal liability)
 (5)  Minimizing the potential for negative exposure
 As in any set of pre-planned procedures, attention must be paid to a
 set of goals for handling an incident.  These goals will be
 prioritized differently depending on the site.  A specific set of
 objectives can be identified for dealing with incidents:
 (1)  Figure out how it happened.
 (2)  Find out how to avoid further exploitation of the same
 (3)  Avoid escalation and further incidents.
 (4)  Assess the impact and damage of the incident.
 (5)  Recover from the incident.
 (6)  Update policies and procedures as needed.
 (7)  Find out who did it (if appropriate and possible).
 Due to the nature of the incident, there might be a conflict between
 analyzing the original source of a problem and restoring systems and
 services.  Overall goals (like assuring the integrity of critical
 systems) might be the reason for not analyzing an incident.  Of
 course, this is an important management decision; but all involved
 parties must be aware that without analysis the same incident may
 happen again.
 It is also important to prioritize the actions to be taken during an
 incident well in advance of the time an incident occurs.  Sometimes
 an incident may be so complex that it is impossible to do everything
 at once to respond to it; priorities are essential.  Although
 priorities will vary from institution to institution, the following
 suggested priorities may serve as a starting point for defining your
 organization's response:
 (1)  Priority one -- protect human life and people's
      safety; human life always has precedence over all
      other considerations.
 (2)  Priority two -- protect classified and/or sensitive
      data.  Prevent exploitation of classified and/or
      sensitive systems, networks or sites.  Inform affected

Fraser, Ed. Informational [Page 40] RFC 2196 Site Security Handbook September 1997

      classified and/or sensitive systems, networks or sites
      about already occurred penetrations.
      (Be aware of regulations by your site or by government)
 (3)  Priority three -- protect other data, including
      proprietary, scientific, managerial and other data,
      because loss of data is costly in terms of resources.
      Prevent exploitations of other systems, networks or
      sites and inform already affected systems, networks or
      sites about successful penetrations.
 (4)  Priority four -- prevent damage to systems (e.g., loss
      or alteration of system files, damage to disk drives,
      etc.).  Damage to systems can result in costly down
      time and recovery.
 (5)  Priority five -- minimize disruption of computing
      resources (including processes).  It is better in many
      cases to shut a system down or disconnect from a network
      than to risk damage to data or systems. Sites will have
      to evaluate the trade-offs between shutting down and
      disconnecting, and staying up. There may be service
      agreements in place that may require keeping systems
      up even in light of further damage occurring. However,
      the damage and scope of an incident may be so extensive
      that service agreements may have to be over-ridden.
 An important implication for defining priorities is that once human
 life and national security considerations have been addressed, it is
 generally more important to save data than system software and
 hardware.  Although it is undesirable to have any damage or loss
 during an incident, systems can be replaced. However, the loss or
 compromise of data (especially classified or proprietary data) is
 usually not an acceptable outcome under any circumstances.
 Another important concern is the effect on others, beyond the systems
 and networks where the incident occurs.  Within the limits imposed by
 government regulations it is always important to inform affected
 parties as soon as possible.  Due to the legal implications of this
 topic, it should be included in the planned procedures to avoid
 further delays and uncertainties for the administrators.
 Any plan for responding to security incidents should be guided by
 local policies and regulations.  Government and private sites that
 deal with classified material have specific rules that they must

Fraser, Ed. Informational [Page 41] RFC 2196 Site Security Handbook September 1997

 The policies chosen by your site on how it reacts to incidents will
 shape your response.  For example, it may make little sense to create
 mechanisms to monitor and trace intruders if your site does not plan
 to take action against the intruders if they are caught.  Other
 organizations may have policies that affect your plans.  Telephone
 companies often release information about telephone traces only to
 law enforcement agencies.
 Handling incidents can be tedious and require any number of routine
 tasks that could be handled by support personnel. To free the
 technical staff it may be helpful to identify support staff who will
 help with tasks like: photocopying, fax'ing, etc.

5.2 Notification and Points of Contact

 It is important to establish contacts with various personnel before a
 real incident occurs.  Many times, incidents are not real
 emergencies. Indeed, often you will be able to handle the activities
 internally. However, there will also be many times when others
 outside your immediate department will need to be included in the
 incident handling.  These additional contacts include local managers
 and system administrators, administrative contacts for other sites on
 the Internet, and various investigative organizations.  Getting to
 know these contacts before incidents occurs will help to make your
 incident handling process more efficient.
 For each type of communication contact, specific "Points of Contact"
 (POC) should be defined.  These may be technical or administrative in
 nature and may include legal or investigative agencies as well as
 service providers and vendors.  When establishing these contact, it
 is important to decide how much information will be shared with each
 class of contact. It is especially important to define, ahead of
 time, what information will be shared with the users at a site, with
 the public (including the press), and with other sites.
 Settling these issues are especially important for the local person
 responsible for handling the incident, since that is the person
 responsible for the actual notification of others.  A list of
 contacts in each of these categories is an important time saver for
 this person during an incident.  It can be quite difficult to find an
 appropriate person during an incident when many urgent events are
 ongoing.  It is strongly recommended that all relevant telephone
 numbers (also electronic mail addresses and fax numbers) be included
 in the site security policy.  The names and contact information of
 all individuals who will be directly involved in the handling of an
 incident should be placed at the top of this list.

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5.2.1 Local Managers and Personnel

 When an incident is under way, a major issue is deciding who is in
 charge of coordinating the activity of the multitude of players.  A
 major mistake that can be made is to have a number of people who are
 each working independently, but are not working together.  This will
 only add to the confusion of the event and will probably lead to
 wasted or ineffective effort.
 The single POC may or may not be the person responsible for handling
 the incident.  There are two distinct roles to fill when deciding who
 shall be the POC and who will be the person in charge of the
 incident.  The person in charge of the incident will make decisions
 as to the interpretation of policy applied to the event.  In
 contrast, the POC must coordinate the effort of all the parties
 involved with handling the event.
 The POC must be a person with the technical expertise to successfully
 coordinate the efforts of the system managers and users involved in
 monitoring and reacting to the attack. Care should be taken when
 identifying who this person will be.  It should not necessarily be
 the same person who has administrative responsibility for the
 compromised systems since often such administrators have knowledge
 only sufficient for the day to day use of the computers, and lack in
 depth technical expertise.
 Another important function of the POC is to maintain contact with law
 enforcement and other external agencies to assure that multi-agency
 involvement occurs.  The level of involvement will be determined by
 management decisions as well as legal constraints.
 A single POC should also be the single person in charge of collecting
 evidence, since as a rule of thumb, the more people that touch a
 potential piece of evidence, the greater the possibility that it will
 be inadmissible in court. To ensure that evidence will be acceptable
 to the legal community, collecting evidence should be done following
 predefined procedures in accordance with local laws and legal
 One of the most critical tasks for the POC is the coordination of all
 relevant processes.  Responsibilities may be distributed over the
 whole site, involving multiple independent departments or groups.
 This will require a  well coordinated effort in order to achieve
 overall success.  The situation becomes even more complex if multiple
 sites are involved.  When this happens, rarely will a single POC at
 one site be able to adequately coordinate the handling of the entire
 incident.  Instead, appropriate incident response teams should be

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 The incident handling process should provide some escalation
 mechanisms.  In order to define such a mechanism, sites will need to
 create an internal classification scheme for incidents. Associated
 with each level of incident will be the appropriate POC and
 procedures.  As an incident is escalated, there may be a change in
 the POC which will need to be communicated to all others involved in
 handling the incident. When a change in the POC occurs, old POC
 should brief the new POC in all background information.
 Lastly, users must know how to report suspected incidents. Sites
 should establish reporting procedures that will work both during and
 outside normal working hours. Help desks are often used to receive
 these reports during normal working hours, while beepers and
 telephones can be used for out of hours reporting.

5.2.2 Law Enforcement and Investigative Agencies

 In the event of an incident that has legal consequences, it is
 important to establish contact with investigative agencies (e.g, the
 FBI and Secret Service in the U.S.) as soon as possible.  Local law
 enforcement, local security offices, and campus police departments
 should also be informed as appropriate.   This section describes many
 of the issues that will be confronted, but it is acknowledged that
 each organization will have its own local and governmental laws and
 regulations that will impact how they interact with law enforcement
 and investigative agencies. The most important point to make is that
 each site needs to work through these issues.
 A primary reason for determining these point of contact well in
 advance of an incident is that once a major attack is in progress,
 there is little time to call these agencies to determine exactly who
 the correct point of contact is.  Another reason is that it is
 important to cooperate with these agencies in a manner that will
 foster a good working relationship, and that will be in accordance
 with the working procedures of these agencies.  Knowing the working
 procedures in advance, and the expectations of your point of contact
 is a big step in this direction.  For example, it is important to
 gather evidence that will be admissible in any subsequent legal
 proceedings, and this will require prior knowledge of how to gather
 such evidence.  A final reason for establishing contacts as soon as
 possible is that it is impossible to know the particular agency that
 will assume jurisdiction in any given incident.  Making contacts and
 finding the proper channels early on will make responding to an
 incident go considerably more smoothly.

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 If your organization or site has a legal counsel, you need to notify
 this office soon after you learn that an incident is in progress.  At
 a minimum, your legal counsel needs to be involved to protect the
 legal and financial interests of your site or organization.  There
 are many legal and practical issues, a few of which are:
 (1)  Whether your site or organization is willing to risk negative
      publicity or exposure to cooperate with legal prosecution
 (2)  Downstream liability--if you leave a compromised system as is so
      it can be monitored and another computer is damaged because the
      attack originated from your system, your site or organization
      may be liable for damages incurred.
 (3)  Distribution of information--if your site or organization
      distributes information about an attack in which another site or
      organization may be involved or the vulnerability in a product
      that may affect ability to market that product, your site or
      organization may again be liable for any damages (including
      damage of reputation).
 (4)  Liabilities due to monitoring--your site or organization may be
      sued if users at your site or elsewhere discover that your site
      is monitoring account activity without informing users.
 Unfortunately, there are no clear precedents yet on the liabilities
 or responsibilities of organizations involved in a security incident
 or who might be involved in supporting an investigative effort.
 Investigators will often encourage organizations to help trace and
 monitor intruders.  Indeed, most investigators cannot pursue computer
 intrusions without extensive support from the organizations involved.
 However, investigators cannot provide protection from liability
 claims, and these kinds of efforts may drag out for months and may
 take a lot of effort.
 On the other hand, an organization's legal council may advise extreme
 caution and suggest that tracing activities be halted and an intruder
 shut out of the system.  This, in itself, may not provide protection
 from liability, and may prevent investigators from identifying the
 The balance between supporting investigative activity and limiting
 liability is tricky. You'll need to consider the advice of your legal
 counsel and the damage the intruder is causing (if any) when making
 your decision about what to do during any particular incident.

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 Your legal counsel should also be involved in any decision to contact
 investigative agencies when an incident occurs at your site.  The
 decision to coordinate efforts with investigative agencies is most
 properly that of your site or organization.  Involving your legal
 counsel will also foster the multi-level coordination between your
 site and the particular investigative agency involved, which in turn
 results in an efficient division of labor.  Another result is that
 you are likely to obtain guidance that will help you avoid future
 legal mistakes.
 Finally, your legal counsel should evaluate your site's written
 procedures for responding to incidents.  It is essential to obtain a
 "clean bill of health" from a legal perspective before you actually
 carry out these procedures.
 It is vital, when dealing with investigative agencies, to verify that
 the person who calls asking for information is a legitimate
 representative from the agency in question.  Unfortunately, many well
 intentioned people have unknowingly leaked sensitive details about
 incidents, allowed unauthorized people into their systems, etc.,
 because a caller has masqueraded as a representative of a government
 agency. (Note: this word of caution actually applies to all external
 A similar consideration is using a secure means of communication.
 Because many network attackers can easily re-route electronic mail,
 avoid using electronic mail to communicate with other agencies (as
 well as others dealing with the incident at hand). Non-secured phone
 lines (the phones normally used in the business world) are also
 frequent targets for tapping by network intruders, so be careful!
 There is no one established set of rules for responding to an
 incident when the local government becomes involved.  Normally (in
 the U.S.), except by legal order, no agency can force you to monitor,
 to disconnect from the network, to avoid telephone contact with the
 suspected attackers, etc. Each organization will have a set of local
 and national laws and regulations that must be adhered to when
 handling incidents. It is recommended that each site be familiar with
 those laws and regulations, and identify and get know the contacts
 for agencies with jurisdiction well in advance of handling an

5.2.3 Computer Security Incident Handling Teams

 There are currently a number of of Computer Security Incident
 Response teams (CSIRTs) such as the CERT Coordination Center, the
 German DFN-CERT, and other teams around the globe.  Teams exist for
 many major government agencies and large corporations.  If such a

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 team is available, notifying it should be of primary consideration
 during the early stages of an incident.  These teams are responsible
 for coordinating computer security incidents over a range of sites
 and larger entities.  Even if the incident is believed to be
 contained within a single site, it is possible that the information
 available through a response team could help in fully resolving the
 If it is determined that the breach occurred due to a flaw in the
 system's hardware or software, the vendor (or supplier) and a
 Computer Security Incident Handling team should be notified as soon
 as possible.  This is especially important because many other systems
 are vulnerable, and these vendor and response team organizations can
 help disseminate help to other affected sites.
 In setting up a site policy for incident handling, it may be
 desirable to create a subgroup, much like those teams that already
 exist, that will be responsible for handling computer security
 incidents for the site (or organization).  If such a team is created,
 it is essential that communication lines be opened between this team
 and other teams.  Once an incident is under way, it is difficult to
 open a trusted dialogue between other teams if none has existed

5.2.4 Affected and Involved Sites

 If an incident has an impact on other sites, it is good practice to
 inform them.  It may be obvious from the beginning that the incident
 is not limited to the local site, or it may emerge only after further
 Each site may choose to contact other sites directly or they can pass
 the information to an appropriate incident response team. It is often
 very difficult to find the responsible POC at remote sites and the
 incident response team will be able to  facilitate contact by making
 use of already established channels.
 The legal and liability issues arising from a security incident will
 differ from site to site.  It is important to define a policy for the
 sharing and logging of information about other sites before an
 incident occurs.
 Information about specific people is especially sensitive, and may be
 subject to privacy laws.  To avoid problems in this area, irrelevant
 information should be deleted and a statement of how to handle the
 remaining information should be included.  A clear statement of how
 this information is to be used is essential.  No one who informs a
 site of a security incident wants to read about it in the public

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 press.  Incident response teams are valuable in this respect.  When
 they pass information to responsible POCs, they are able to protect
 the anonymity of the original source. But, be aware that, in many
 cases, the analysis of logs and information at other sites will
 reveal addresses of your site.
 All the problems discussed above should be not taken as reasons not
 to involve other sites.  In fact, the experiences of existing teams
 reveal that most sites informed about security problems are not even
 aware that their site had been compromised.  Without timely
 information, other sites are often unable to take action against

5.2.5 Internal Communications

 It is crucial during a major incident to communicate why certain
 actions are being taken, and how the users (or departments) are
 expected to behave. In particular, it should be made very clear to
 users what they are allowed to say (and not say) to the outside world
 (including other departments). For example, it wouldn't be good for
 an organization if users replied to customers with something like,
 "I'm sorry the systems are down, we've had an intruder and we are
 trying to clean things up." It would be much better if they were
 instructed to respond with a prepared statement like, "I'm sorry our
 systems are unavailable, they are being maintained for better service
 in the future."
 Communications with customers and contract partners should be handled
 in a sensible, but sensitive way. One can prepare for the main issues
 by preparing a checklist. When an incident occurs, the checklist can
 be used with the addition of a sentence or two for the specific
 circumstances of the incident.
 Public relations departments can be very helpful during incidents.
 They should be involved in all planning and can provide well
 constructed responses for use when contact with outside departments
 and organizations is necessary.

5.2.6 Public Relations - Press Releases

 There has been a tremendous growth in the amount of media coverage
 dedicated to computer security incidents in the United States. Such
 press coverage is bound to extend to other countries as the Internet
 continues to grow and expand internationally.  Readers from countries
 where such media attention has not yet occurred, can learn from the
 experiences in the U.S. and should be forwarned and prepared.

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 One of the most important issues to consider is when, who, and how
 much to release to the general public through the press.  There are
 many issues to consider when deciding this particular issue.  First
 and foremost, if a public relations office exists for the site, it is
 important to use this office as liaison to the press.  The public
 relations office is trained in the type and wording of information
 released, and will help to assure that the image of the site is
 protected during and after the incident (if possible).  A public
 relations office has the advantage that you can communicate candidly
 with them, and provide a buffer between the constant press attention
 and the need of the POC to maintain control over the incident.
 If a public relations office is not available, the information
 released to the press must be carefully considered.  If the
 information is sensitive, it may be advantageous to provide only
 minimal or overview information to the press.  It is quite possible
 that any information provided to the press will be quickly reviewed
 by the perpetrator of the incident.  Also note that misleading the
 press can often backfire and cause more damage than releasing
 sensitive information.
 While it is difficult to determine in advance what level of detail to
 provide to the press, some guidelines to keep in mind are:
 (1)  Keep the technical level of detail low.  Detailed
      information about the incident may provide enough
      information for others to launch similar attacks on
      other sites, or even damage the site's ability to
      prosecute the guilty party once the event is over.
 (2)  Keep the speculation out of press statements.
      Speculation of who is causing the incident or the
      motives are very likely to be in error and may cause
      an inflamed view of the incident.
 (3)  Work with law enforcement professionals to assure that
      evidence is protected.  If prosecution is involved,
      assure that the evidence collected is not divulged to
      the press.
 (4)  Try not to be forced into a press interview before you are
      prepared.  The popular press is famous for the "2 am"
      interview, where the hope is to catch the interviewee off
      guard and obtain information otherwise not available.
 (5)  Do not allow the press attention to detract from the
      handling of the event.  Always remember that the successful
      closure of an incident is of primary importance.

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5.3 Identifying an Incident

5.3.1 Is It Real?

 This stage involves determining if a problem really exists.  Of
 course many if not most signs often associated with virus infection,
 system intrusions, malicious users, etc., are simply anomalies such
 as hardware failures or suspicious system/user behavior.  To assist
 in identifying whether there really is an incident, it is usually
 helpful to obtain and use any detection software which may be
 available.  Audit information is also extremely useful, especially in
 determining whether there is a network attack.  It is extremely
 important to obtain a system snapshot as soon as one suspects that
 something is wrong.  Many incidents cause a dynamic chain of events
 to occur, and an initial system snapshot may be the most valuable
 tool for identifying the problem and any source of attack.  Finally,
 it is important to start a log book.  Recording system events,
 telephone conversations, time stamps, etc., can lead to a more rapid
 and systematic identification of the problem, and is the basis for
 subsequent stages of incident handling.
 There are certain indications or "symptoms" of an incident that
 deserve special attention:
 (1)   System crashes.
 (2)   New user accounts (the account RUMPLESTILTSKIN has been
       unexpectedly created), or high activity on a previously
       low usage account.
 (3)   New files (usually with novel or strange file names,
       such as data.xx or k or .xx ).
 (4)   Accounting discrepancies (in a UNIX system you might
       notice the shrinking of an accounting file called
       /usr/admin/lastlog, something that should make you very
       suspicious that there may be an intruder).
 (5)   Changes in file lengths or dates (a user should be
       suspicious if .EXE files in an MS DOS computer have
       unexplainedly grown by over 1800 bytes).
 (6)   Attempts to write to system (a system manager notices
       that a privileged user in a VMS system is attempting to
       alter RIGHTSLIST.DAT).
 (7)   Data modification or deletion (files start to disappear).
 (8)   Denial of service (a system manager and all other users
       become locked out of a UNIX system, now in single user mode).
 (9)   Unexplained, poor system performance
 (10)  Anomalies ("GOTCHA" is displayed on the console or there
       are frequent unexplained "beeps").
 (11)  Suspicious probes (there are numerous unsuccessful login
       attempts from another node).

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 (12)  Suspicious browsing (someone becomes a root user on a UNIX
       system and accesses file after file on many user accounts.)
 (13)  Inability of a user to log in due to modifications of his/her
 By no means is this list comprehensive; we have just listed a number
 of common indicators.  It is best to collaborate with other technical
 and computer security personnel to make a decision as a group about
 whether an incident is occurring.

5.3.2 Types and Scope of Incidents

 Along with the identification of the incident is the evaluation of
 the scope and impact of the problem.  It is important to correctly
 identify the boundaries of the incident in order to effectively deal
 with it and prioritize responses.
 In order to identify the scope and impact a set of criteria should be
 defined which is appropriate to the site and to the type of
 connections available.  Some of the issues include:
 (1)  Is this a multi-site incident?
 (2)  Are many computers at your site affected by this incident?
 (3)  Is sensitive information involved?
 (4)  What is the entry point of the incident (network,
      phone line, local terminal, etc.)?
 (5)  Is the press involved?
 (6)  What is the potential damage of the incident?
 (7)  What is the estimated time to close out the incident?
 (8)  What resources could be required to handle the incident?
 (9)  Is law enforcement involved?

5.3.3 Assessing the Damage and Extent

 The analysis of the damage and extent of the incident can be quite
 time consuming, but should lead to some insight into the nature of
 the incident, and aid investigation and prosecution.  As soon as the
 breach has occurred, the entire system and all of its components
 should be considered suspect.  System software is the most probable
 target.  Preparation is key to be able to detect all changes for a
 possibly tainted system.  This includes checksumming all media from
 the vendor using a algorithm which is resistant to tampering.  (See
 sections 4.3)
 Assuming original vendor distribution media are available, an
 analysis of all system files should commence, and any irregularities
 should be noted and referred to all parties involved in handling the
 incident.  It can be very difficult, in some cases, to decide which

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 backup media are showing a correct system status. Consider, for
 example, that the incident may have continued for months or years
 before discovery, and the suspect may be an employee of the site, or
 otherwise have intimate knowledge or access to the systems.  In all
 cases, the pre-incident preparation will determine what recovery is
 If the system supports centralized logging (most do), go back over
 the logs and look for abnormalities.  If process accounting and
 connect time accounting is enabled, look for patterns of system
 usage.  To a lesser extent, disk usage may shed light on the
 incident.  Accounting can provide much helpful information in an
 analysis of an incident and subsequent prosecution.  Your ability to
 address all aspects of a specific incident strongly depends on the
 success of this analysis.

5.4 Handling an Incident

 Certain steps are necessary to take during the handling of an
 incident.  In all security related activities, the most important
 point to be made is that all sites should have policies in place.
 Without defined policies and goals, activities undertaken will remain
 without focus. The goals should be defined by management and legal
 counsel in advance.
 One of the most fundamental objectives is to restore control of the
 affected systems and to limit the impact and damage.  In the worst
 case scenario, shutting down the system, or disconnecting the system
 from the network, may the only practical solution.
 As the activities involved are complex, try to get as much help as
 necessary.  While trying to solve the problem alone, real damage
 might occur due to delays or missing information.  Most
 administrators take the discovery of an intruder as a personal
 challenge.  By proceeding this way, other objectives as outlined in
 the local policies may not always be considered.  Trying to catch
 intruders may be a very low priority, compared to system integrity,
 for example.  Monitoring a hacker's activity is useful, but it might
 not be considered worth the risk to allow the continued access.

5.4.1 Types of Notification and Exchange of Information

 When you have confirmed that an incident is occurring, the
 appropriate personnel must be notified.  How this notification is
 achieved is very important to keeping the event under control both
 from a technical and emotional standpoint. The circumstances should
 be described in as much detail as possible, in order to aid prompt
 acknowledgment and understanding of the problem.  Great care should

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 be taken when determining to which groups detailed technical
 information is given during the notification.  For example, it is
 helpful to pass this kind of information to an incident handling team
 as they can assist you by providing helpful hints for eradicating the
 vulnerabilities involved in an incident.  On the other hand, putting
 the critical knowledge into the public domain (e.g., via USENET
 newsgroups or mailing lists) may potentially put a large number of
 systems at risk of intrusion.  It is invalid to assume that all
 administrators reading a particular newsgroup have access to
 operating system source code, or can even understand an advisory well
 enough to take adequate steps.
 First of all, any notification to either local or off-site personnel
 must be explicit.  This requires that any statement (be it an
 electronic mail message, phone call, fax, beeper, or semaphone)
 providing information about the incident be clear, concise, and fully
 qualified.  When you are notifying others that will help you handle
 an event, a "smoke screen" will only divide the effort and create
 confusion.  If a division of labor is suggested, it is helpful to
 provide information to each participant about what is being
 accomplished in other efforts.  This will not only reduce duplication
 of effort, but allow people working on parts of the problem to know
 where to obtain information relevant to their part of the incident.
 Another important consideration when communicating about the incident
 is to be factual.  Attempting to hide aspects of the incident by
 providing false or incomplete information may not only prevent a
 successful resolution to the incident, but may even worsen the
 The choice of language used when notifying people about the incident
 can have a profound effect on the way that information is received.
 When you use emotional or inflammatory terms, you raise the potential
 for damage and negative outcomes of the incident.  It is important to
 remain calm both in written and spoken communications.
 Another consideration is that not all people speak the same language.
 Due to this fact, misunderstandings and delay may arise, especially
 if it is a multi-national incident. Other international concerns
 include differing legal implications of a security incident and
 cultural differences.  However, cultural differences do not only
 exist between countries.  They even exist within countries, between
 different social or user groups.  For example, an administrator of a
 university system might be very relaxed about attempts to connect to
 the system via telnet, but the administrator of a military system is
 likely to consider the same action as a possible attack.

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 Another issue associated with the choice of language is the
 notification of non-technical or off-site personnel.  It is important
 to accurately describe the incident without generating undue alarm or
 confusion.  While it is more difficult to describe the incident to a
 non-technical audience, it is often more important.  A non-technical
 description may be required for upper-level management, the press, or
 law enforcement liaisons.  The importance of these communications
 cannot be underestimated and may make the difference between
 resolving the incident properly and escalating to some higher level
 of damage.
 If an incident response team becomes involved, it might be necessary
 to fill out a template for the information exchange.  Although this
 may seem to be an additional burden and adds a certain delay, it
 helps the team to act on this minimum set of information.  The
 response team may be able to respond to aspects of the incident of
 which the local administrator is unaware. If information is given out
 to someone else, the following minimum information should be
 (1)  timezone of logs, ... in GMT or local time
 (2)  information about the remote system, including host names,
      IP addresses and (perhaps) user IDs
 (3)  all log entries relevant for the remote site
 (4)  type of incident (what happened, why should you care)
 If local information (i.e., local user IDs) is included in the log
 entries, it will be necessary to sanitize the entries beforehand to
 avoid privacy issues.  In general, all information which might assist
 a remote site in resolving an incident should be given out, unless
 local policies prohibit this.

5.4.2 Protecting Evidence and Activity Logs

 When you respond to an incident, document all details related to the
 incident.  This will provide valuable information to yourself and
 others as you try to unravel the course of events.  Documenting all
 details will ultimately save you time.  If you don't document every
 relevant phone call, for example, you are likely to forget a
 significant portion of information you obtain, requiring you to
 contact the source of information again.  At the same time, recording
 details will provide evidence for prosecution efforts, providing the
 case moves in that direction.  Documenting an incident will also help
 you perform a final assessment of damage (something your management,
 as well as law enforcement officers, will want to know), and will
 provide the basis for later phases of the handling process:
 eradication, recovery, and follow-up "lessons learned."

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 During the initial stages of an incident, it is often infeasible to
 determine whether prosecution is viable, so you should document as if
 you are gathering evidence for a court case.  At a minimum, you
 should record:
 (1)  all system events (audit records)
 (2)  all actions you take (time tagged)
 (3)  all external conversations (including the person with whom
      you talked, the date and time, and the content of the
 The most straightforward way to maintain documentation is keeping a
 log book.  This allows you to go to a centralized, chronological
 source of information when you need it, instead of requiring you to
 page through individual sheets of paper.  Much of this information is
 potential evidence in a court of law.  Thus, when a legal follow-up
 is a possibility, one should follow the prepared procedures and avoid
 jeopardizing the legal follow-up by improper handling of possible
 evidence. If appropriate, the following steps may be taken.
 (1)  Regularly (e.g., every day) turn in photocopied, signed
      copies of your logbook (as well as media you use to record
      system events) to a document custodian.
 (2)  The custodian should store these copied pages in a secure
      place (e.g., a safe).
 (3)  When you submit information for storage, you should
      receive a signed, dated receipt from the document
 Failure to observe these procedures can result in invalidation of any
 evidence you obtain in a court of law.

5.4.3 Containment

 The purpose of containment is to limit the extent of an attack.  An
 essential part of containment is decision making (e.g., determining
 whether to shut a system down, disconnect from a network, monitor
 system or network activity, set traps, disable functions such as
 remote file transfer, etc.).
 Sometimes this decision is trivial; shut the system down if the
 information is classified, sensitive, or proprietary.  Bear in mind
 that removing all access while an incident is in progress obviously
 notifies all users, including the alleged problem users, that the
 administrators are aware of a problem; this may have a deleterious

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 effect on an investigation.  In some cases, it is prudent to remove
 all access or functionality as soon as possible, then restore normal
 operation in limited stages.  In other cases, it is worthwhile to
 risk some damage to the system if keeping the system up might enable
 you to identify an intruder.
 This stage should involve carrying out predetermined procedures.
 Your organization or site should, for example, define acceptable
 risks in dealing with an incident, and should prescribe specific
 actions and strategies accordingly.  This is especially important
 when a quick decision is necessary and it is not possible to first
 contact all involved parties to discuss the decision.  In the absence
 of predefined procedures, the person in charge of the incident will
 often not have the power to make difficult management decisions (like
 to lose the results of a costly experiment by shutting down a
 system).  A final activity that should occur during this stage of
 incident handling is the notification of appropriate authorities.

5.4.4 Eradication

 Once the incident has been contained, it is time to eradicate the
 cause.  But before eradicating the cause, great care should be taken
 to collect all necessary information about the compromised system(s)
 and the cause of the incident as they will likely be lost when
 cleaning up the system.
 Software may be available to help you in the eradication process,
 such as anti-virus software.  If any bogus files have been created,
 archive them before deleting them.  In the case of virus infections,
 it is important to clean and reformat any media containing infected
 files.  Finally, ensure that all backups are clean.  Many systems
 infected with viruses become periodically re-infected simply because
 people do not systematically eradicate the virus from backups.  After
 eradication, a new backup should be taken.
 Removing all vulnerabilities once an incident has occurred is
 difficult.  The key to removing vulnerabilities is knowledge and
 understanding of the breach.
 It may be necessary to go back to the original distribution media and
 re-customize the system.  To facilitate this worst case scenario, a
 record of the original system setup and each customization change
 should be maintained.  In the case of a network-based attack, it is
 important to install patches for each operating system vulnerability
 which was exploited.

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 As discussed in section 5.4.2, a security log can be most valuable
 during this phase of removing vulnerabilities. The logs showing how
 the incident was discovered and contained can be used later to help
 determine how extensive the damage was from a given incident.  The
 steps taken can be used in the future to make sure the problem does
 not resurface.  Ideally, one should automate and regularly apply the
 same test as was used to detect the security incident.
 If a particular vulnerability is isolated as having been exploited,
 the next step is to find a mechanism to protect your system.  The
 security mailing lists and bulletins would be a good place to search
 for this information, and you can get advice from incident response

5.4.5 Recovery

 Once the cause of an incident has been eradicated, the recovery phase
 defines the next stage of action.  The goal of recovery is to return
 the system to normal.  In general, bringing up services in the order
 of demand to allow a minimum of user inconvenience is the best
 practice.  Understand that the proper recovery procedures for the
 system are extremely important and should be specific to the site.

5.4.6 Follow-Up

 Once you believe that a system has been restored to a "safe" state,
 it is still possible that holes, and even traps, could be lurking in
 the system.  One of the most important stages of responding to
 incidents is also the most often omitted, the follow-up stage.  In
 the follow-up stage, the system should be monitored for items that
 may have been missed during the cleanup stage.  It would be prudent
 to utilize some of the tools mentioned in chapter 7 as a start.
 Remember, these tools don't replace continual system monitoring and
 good systems administration practices.
 The most important element of the follow-up stage is performing a
 postmortem analysis.  Exactly what happened, and at what times?  How
 well did the staff involved with the incident perform?  What kind of
 information did the staff need quickly, and how could they have
 gotten that information as soon as possible?  What would the staff do
 differently next time?
 After an incident, it is prudent to write a report describing the
 exact sequence of events: the method of discovery, correction
 procedure, monitoring procedure, and a summary of lesson learned.
 This will aid in the clear understanding of the problem.  Creating a
 formal chronology of events (including time stamps) is also important
 for legal reasons.

Fraser, Ed. Informational [Page 57] RFC 2196 Site Security Handbook September 1997

 A follow-up report is valuable for many reasons.  It provides a
 reference to be used in case of other similar incidents.  It is also
 important to, as quickly as possible obtain a monetary estimate of
 the amount of damage the incident caused. This estimate should
 include costs associated with any loss of software and files
 (especially the value of proprietary data that may have been
 disclosed), hardware damage, and manpower costs to restore altered
 files, reconfigure affected systems, and so forth.  This estimate may
 become the basis for subsequent prosecution activity.  The report can
 also help justify an organization's computer security effort to

5.5 Aftermath of an Incident

 In the wake of an incident, several actions should take place.  These
 actions can be summarized as follows:
 (1)  An inventory should be taken of the systems' assets,
      (i.e., a careful examination should determine how the
      system was affected by the incident).
 (2)  The lessons learned as a result of the incident
      should be included in revised security plan to
      prevent the incident from re-occurring.
 (3)  A new risk analysis should be developed in light of the
 (4)  An investigation and prosecution of the individuals
      who caused the incident should commence, if it is
      deemed desirable.
 If an incident is based on poor policy, and unless the policy is
 changed, then one is doomed to repeat the past.  Once a site has
 recovered from and incident, site policy and procedures should be
 reviewed to encompass changes to prevent similar incidents.  Even
 without an incident, it would be prudent to review policies and
 procedures on a regular basis.  Reviews are imperative due to today's
 changing computing environments.
 The whole purpose of this post mortem process is to improve all
 security measures to protect the site against future attacks.  As a
 result of an incident, a site or organization should gain practical
 knowledge from the experience.  A concrete goal of the post mortem is
 to develop new proactive methods.  Another important facet of the
 aftermath may be end user and administrator education to prevent a
 reoccurrence of the security problem.

Fraser, Ed. Informational [Page 58] RFC 2196 Site Security Handbook September 1997

5.6 Responsibilities

5.6.1 Not Crossing the Line

 It is one thing to protect one's own network, but quite another to
 assume that one should protect other networks.  During the handling
 of an incident, certain system vulnerabilities of one's own systems
 and the systems of others become apparent.  It is quite easy and may
 even be tempting to pursue the intruders in order to track them.
 Keep in mind that at a certain point it is possible to "cross the
 line," and, with the best of intentions, become no better than the
 The best rule when it comes to propriety is to not use any facility
 of remote sites which is not public.  This clearly excludes any entry
 onto a system (such as a remote shell or login session) which is not
 expressly permitted.  This may be very tempting; after a breach of
 security is detected, a system administrator may have the means to
 "follow it up," to ascertain what damage is being done to the remote
 site.  Don't do it!  Instead, attempt to reach the appropriate point
 of contact for the affected site.

5.6.2 Good Internet Citizenship

 During a security incident there are two choices one can make.
 First, a site can choose to watch the intruder in the hopes of
 catching him; or, the site can go about cleaning up after the
 incident and shut the intruder out of the systems.  This is a
 decision that must be made very thoughtfully, as there may be legal
 liabilities if you choose to leave your site open, knowing that an
 intruder is using your site as a launching pad to reach out to other
 sites.  Being a good Internet citizen means that you should try to
 alert other sites that may have been impacted by the intruder.  These
 affected sites may be readily apparent after a thorough review of
 your log files.

5.6.3 Administrative Response to Incidents

 When a security incident involves a user, the site's security policy
 should describe what action is to be taken.  The transgression should
 be taken seriously, but it is very important to be sure of the role
 the user played.  Was the user naive?  Could there be a mistake in
 attributing the security breach to the user?  Applying administrative
 action that assumes the user intentionally caused the incident may

Fraser, Ed. Informational [Page 59] RFC 2196 Site Security Handbook September 1997

 not be appropriate for a user who simply made a mistake.  It may be
 appropriate to include sanctions more suitable for such a situation
 in your policies (e.g., education or reprimand of a user) in addition
 to more stern measures for intentional acts of intrusion and system

6. Ongoing Activities

 At this point in time, your site has hopefully developed a complete
 security policy and has developed procedures to assist in the
 configuration and management of your technology in support of those
 policies.  How nice it would be if you could sit back and relax at
 this point and know that you were finished with the job of security.
 Unfortunately, that isn't possible.  Your systems and networks are
 not a static environment, so you will need to review policies and
 procedures on a regular basis.  There are a number of steps you can
 take to help you keep up with the changes around you so that you can
 initiate corresponding actions to address those changes.  The
 following is a starter set and you may add others as appropriate for
 your site.
 (1)  Subscribe to advisories that are issued by various security incident
      response teams, like those of the CERT Coordination Center, and
      update your systems against those threats that apply to your site's
 (2)  Monitor security patches that are produced by the vendors of your
      equipment, and obtain and install all that apply.
 (3)  Actively watch the configurations of your systems to identify any
      changes that may have occurred, and investigate all anomalies.
 (4)  Review all security policies and procedures annually (at a minimum).
 (5)  Read relevant mailing lists and USENET newsgroups to keep up to
      date with the latest information being shared by fellow
 (6)  Regularly check for compliance with policies and procedures.  This
      audit should be performed by someone other than the people who
      define or implement the policies and procedures.

7. Tools and Locations

 This chapter provides a brief list of publicly available security
 technology which can be downloaded from the Internet.  Many of the
 items described below will undoubtedly be surpassed or made obsolete
 before this document is published.

Fraser, Ed. Informational [Page 60] RFC 2196 Site Security Handbook September 1997

 Some of the tools listed are applications such as end user programs
 (clients) and their supporting system infrastructure (servers).
 Others are tools that a general user will never see or need to use,
 but may be used by applications, or by administrators to troubleshoot
 security problems or to guard against intruders.
 A sad fact is that there are very few security conscious applications
 currently available. Primarily, this is caused by the need for a
 security infrastructure which must first be put into place for most
 applications to operate securely.  There is considerable effort
 currently taking place to build this infrastructure so that
 applications can take advantage of secure communications.
 Most of the tools and applications described below can be found in
 one of the following archive sites:
 (1)  CERT Coordination Center
 (2)  DFN-CERT
 (3)  Computer Operations, Audit, and Security Tools (COAST)
 It is important to note that many sites, including CERT and COAST are
 mirrored throughout the Internet.  Be careful to use a "well known"
 mirror site to retrieve software, and to use verification tools (md5
 checksums, etc.) to validate that software.  A clever cracker might
 advertise security software that has intentionally been designed to
 provide access to data or systems.


 identd (not really a security tool)
 rpcbind/portmapper replacement

Fraser, Ed. Informational [Page 61] RFC 2196 Site Security Handbook September 1997


8. Mailing Lists and Other Resources

 It would be impossible to list all of the mail-lists and other
 resources dealing with site security. However, these are some "jump-
 points"  from which the reader can begin. All of these references are
 for the "INTERNET" constituency. More specific (vendor and
 geographical) resources can be found through these references.
 Mailing Lists
 (1)  CERT(TM) Advisory
      Send mail to:
      Message Body:  subscribe cert <FIRST NAME> <LAST NAME>
      A CERT advisory provides information on how to obtain a patch or
      details of a workaround for a known computer security problem.
      The CERT Coordination Center works with vendors to produce a
      workaround or a patch for a problem, and does not publish
      vulnerability information until a workaround or a patch is
      available. A CERT advisory may also be a warning to our
      constituency about ongoing attacks (e.g.,
      CERT advisories are also published on the USENET newsgroup:
      CERT advisory archives are available via anonymous FTP from in the /pub/cert_advisories directory.
 (2)  VIRUS-L List
      Send mail to:
      Message Body:  subscribe virus-L FIRSTNAME LASTNAME
      VIRUS-L is a moderated mailing list with a focus
      on computer virus issues.  For more information,
      including a copy of the posting guidelines, see
      the file "virus-l.README", available by anonymous
      FTP from

Fraser, Ed. Informational [Page 62] RFC 2196 Site Security Handbook September 1997

 (3)  Internet Firewalls
      Send mail to:
      Message Body:  subscribe firewalls user@host
      The Firewalls mailing list is a discussion forum for
      firewall administrators and implementors.
 USENET newsgroups
      The newsgroup is moderated
      and is used solely for the distribution of CERT
      The is a forum for the
      discussion of computer security, especially as it
      relates to the UNIX(r) Operating System.
      The newsgroup is also a forum for the
      discussion of computer security, as well as other
      issues such as car locks and alarm systems.
 (4)  comp.virus
      The comp.virus newsgroup is a moderated newsgroup
      with a focus on computer virus issues.  For more
      information, including a copy of the posting
      guidelines, see the file "virus-l.README",
      available via anonymous FTP on
      in the /pub/virus-l directory.
 (5)  comp.risks
      The comp.risks newsgroup is a moderated forum on
      the risks to the public in computers and related
 World-Wide Web Pages
      Computer Security Resource Clearinghouse. The main focus is on
      crisis response information; information on computer
      security-related threats, vulnerabilities, and solutions. At the
      same time, the Clearinghouse strives to be a general index to
      computer security information on a broad variety of subjects,
      including general risks, privacy, legal issues, viruses,
      assurance, policy, and training.

Fraser, Ed. Informational [Page 63] RFC 2196 Site Security Handbook September 1997

      This Reference Index contains a list of links to information
      sources on Network and Computer Security. There is no implied
      fitness to the Tools, Techniques and Documents contained within this
      archive. Many if not all of these items work well, but we do
      not guarantee that this will be so. This information is for the
      education and legitimate use of computer security techniques only.
      This page features general information about computer security.
      Information is organized by source and each section is organized
      by topic. Recent modifications are noted in What's New page.
      This archive at the National Institute of Standards and Technology's
      Computer Security Resource Clearinghouse page contains a number of
      announcements, programs, and documents related to computer security.
  • CERT and Tripwire are registered in the U.S. Patent and Trademark Office

9. References

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Fraser, Ed. Informational [Page 66] RFC 2196 Site Security Handbook September 1997

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Fraser, Ed. Informational [Page 67] RFC 2196 Site Security Handbook September 1997

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Fraser, Ed. Informational [Page 69] RFC 2196 Site Security Handbook September 1997

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Fraser, Ed. Informational [Page 70] RFC 2196 Site Security Handbook September 1997

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Fraser, Ed. Informational [Page 71] RFC 2196 Site Security Handbook September 1997

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Security Considerations

 This entire document discusses security issues.

Editor Information

 Barbara Y. Fraser
 Software Engineering Institute
 Carnegie Mellon University
 5000 Forbes Avenue
 Pittsburgh, PA 15213
 Phone: (412) 268-5010
 Fax:   (412) 268-6989

Fraser, Ed. Informational [Page 75]

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