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Network Working Group E. Rescorla Request for Comments: 3552 RTFM, Inc. BCP: 72 B. Korver Category: Best Current Practice Xythos Software

                                           Internet Architecture Board
                                                             July 2003
     Guidelines for Writing RFC Text on Security Considerations

Status of this Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.


 All RFCs are required to have a Security Considerations section.
 Historically, such sections have been relatively weak.  This document
 provides guidelines to RFC authors on how to write a good Security
 Considerations section.

Table of Contents

 1. Introduction . . . . . . . . . . . . . . . . . . . . . . .   3
    1.1. Requirements. . . . . . . . . . . . . . . . . . . . .   3
 2. The Goals of Security. . . . . . . . . . . . . . . . . . .   3
    2.1. Communication Security. . . . . . . . . . . . . . . .   3
         2.1.1. Confidentiality. . . . . . . . . . . . . . . .   4
         2.1.2. Data Integrity . . . . . . . . . . . . . . . .   4
         2.1.3. Peer Entity authentication . . . . . . . . . .   4
    2.2. Non-Repudiation . . . . . . . . . . . . . . . . . . .   5
    2.3. Systems Security. . . . . . . . . . . . . . . . . . .   5
         2.3.1. Unauthorized Usage . . . . . . . . . . . . . .   6
         2.3.2. Inappropriate Usage. . . . . . . . . . . . . .   6
         2.3.3. Denial of Service. . . . . . . . . . . . . . .   6
 3. The Internet Threat Model. . . . . . . . . . . . . . . . .   6
    3.1. Limited Threat Models . . . . . . . . . . . . . . . .   7
    3.2. Passive Attacks . . . . . . . . . . . . . . . . . . .   7
         3.2.1. Confidentiality Violations . . . . . . . . . .   8
         3.2.2. Password Sniffing. . . . . . . . . . . . . . .   8
         3.2.3. Offline Cryptographic Attacks. . . . . . . . .   9

Rescorla & Korver Best Current Practice [Page 1] RFC 3552 Security Considerations Guidelines July 2003

    3.3. Active Attacks. . . . . . . . . . . . . . . . . . . .   9
         3.3.1. Replay Attacks . . . . . . . . . . . . . . . .  10
         3.3.2. Message Insertion. . . . . . . . . . . . . . .  10
         3.3.3. Message Deletion . . . . . . . . . . . . . . .  11
         3.3.4. Message Modification . . . . . . . . . . . . .  11
         3.3.5. Man-In-The-Middle. . . . . . . . . . . . . . .  12
    3.4. Topological Issues. . . . . . . . . . . . . . . . . .  12
    3.5. On-path versus off-path . . . . . . . . . . . . . . .  13
    3.6. Link-local. . . . . . . . . . . . . . . . . . . . . .  13
 4. Common Issues. . . . . . . . . . . . . . . . . . . . . . .  13
    4.1. User Authentication . . . . . . . . . . . . . . . . .  14
         4.1.1. Username/Password. . . . . . . . . . . . . . .  14
         4.1.2. Challenge Response and One Time Passwords. . .  14
         4.1.3. Shared Keys. . . . . . . . . . . . . . . . . .  15
         4.1.4. Key Distribution Centers . . . . . . . . . . .  15
         4.1.5. Certificates . . . . . . . . . . . . . . . . .  15
         4.1.6. Some Uncommon Systems. . . . . . . . . . . . .  15
         4.1.7. Host Authentication. . . . . . . . . . . . . .  16
    4.2. Generic Security Frameworks . . . . . . . . . . . . .  16
    4.3. Non-repudiation . . . . . . . . . . . . . . . . . . .  17
    4.4. Authorization vs. Authentication. . . . . . . . . . .  18
         4.4.1. Access Control Lists . . . . . . . . . . . . .  18
         4.4.2. Certificate Based Systems. . . . . . . . . . .  18
    4.5. Providing Traffic Security. . . . . . . . . . . . . .  19
         4.5.1. IPsec. . . . . . . . . . . . . . . . . . . . .  19
         4.5.2. SSL/TLS. . . . . . . . . . . . . . . . . . . .  20
         4.5.3. Remote Login . . . . . . . . . . . . . . . . .  22
    4.6. Denial of Service Attacks and Countermeasures . . . .  22
         4.6.1. Blind Denial of Service. . . . . . . . . . . .  23
         4.6.2. Distributed Denial of Service. . . . . . . . .  23
         4.6.3. Avoiding Denial of Service . . . . . . . . . .  24
         4.6.4. Example: TCP SYN Floods. . . . . . . . . . . .  24
         4.6.5. Example: Photuris. . . . . . . . . . . . . . .  25
    4.7. Object vs. Channel Security . . . . . . . . . . . . .  25
    4.8. Firewalls and Network Topology. . . . . . . . . . . .  26
 5. Writing Security Considerations Sections . . . . . . . . .  26
 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . .  28
    6.1. SMTP. . . . . . . . . . . . . . . . . . . . . . . . .  29
         6.1.1. Security Considerations. . . . . . . . . . . .  29
         6.1.2. Communications security issues . . . . . . . .  34
         6.1.3. Denial of Service. . . . . . . . . . . . . . .  36
    6.2. VRRP. . . . . . . . . . . . . . . . . . . . . . . . . .36
         6.2.1. Security Considerations. . . . . . . . . . . .  36
 7. Acknowledgments. . . . . . . . . . . . . . . . . . . . . .  38
 8. Normative References . . . . . . . . . . . . . . . . . . .  39
 9. Informative References . . . . . . . . . . . . . . . . . .  41
 10.Security Considerations. . . . . . . . . . . . . . . . . .  42
 Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . .  43

Rescorla & Korver Best Current Practice [Page 2] RFC 3552 Security Considerations Guidelines July 2003

 Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  43
 Full Copyright Statement. . . . . . . . . . . . . . . . . . .  44

1. Introduction

 All RFCs are required by RFC 2223 to contain a Security
 Considerations section.  The purpose of this is both to encourage
 document authors to consider security in their designs and to inform
 the reader of relevant security issues.  This memo is intended to
 provide guidance to RFC authors in service of both ends.
 This document is structured in three parts.  The first is a
 combination security tutorial and definition of common terms; the
 second is a series of guidelines for writing Security Considerations;
 the third is a series of examples.

1.1. Requirements

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in BCP 14, RFC 2119

2. The Goals of Security

 Most people speak of security as if it were a single monolithic
 property of a protocol or system, however, upon reflection, one
 realizes that it is clearly not true.  Rather, security is a series
 of related but somewhat independent properties.  Not all of these
 properties are required for every application.
 We can loosely divide security goals into those related to protecting
 communications (COMMUNICATION SECURITY, also known as COMSEC) and
 those relating to protecting systems (ADMINISTRATIVE SECURITY or
 SYSTEM SECURITY).  Since communications are carried out by systems
 and access to systems is through communications channels, these goals
 obviously interlock, but they can also be independently provided.

2.1. Communication Security

 Different authors partition the goals of communication security
 differently.  The partitioning we've found most useful is to divide
 them into three major categories: CONFIDENTIALITY, DATA INTEGRITY and

Rescorla & Korver Best Current Practice [Page 3] RFC 3552 Security Considerations Guidelines July 2003

2.1.1. Confidentiality

 When most people think of security, they think of CONFIDENTIALITY.
 Confidentiality means that your data is kept secret from unintended
 listeners.  Usually, these listeners are simply eavesdroppers.  When
 an adversary taps your phone, it poses a risk to your
 Obviously, if you have secrets, then you are probably concerned about
 others discovering them.  Thus, at the very least, you want to
 maintain confidentiality.  When you see spies in the movies go into
 the bathroom and turn on all the water to foil bugging, the property
 they're looking for is confidentiality.

2.1.2. Data Integrity

 The second primary goal is DATA INTEGRITY.  The basic idea here is
 that we want to make sure that the data we receive is the same data
 that the sender has sent.  In paper-based systems, some data
 integrity comes automatically.  When you receive a letter written in
 pen you can be fairly certain that no words have been removed by an
 attacker because pen marks are difficult to remove from paper.
 However, an attacker could have easily added some marks to the paper
 and completely changed the meaning of the message.  Similarly, it's
 easy to shorten the page to truncate the message.
 On the other hand, in the electronic world, since all bits look
 alike, it's trivial to tamper with messages in transit.  You simply
 remove the message from the wire, copy out the parts you like, add
 whatever data you want, and generate a new message of your choosing,
 and the recipient is no wiser.  This is the moral equivalent of the
 attacker taking a letter you wrote, buying some new paper and
 recopying the message, changing it as he does it.  It's just a lot
 easier to do electronically since all bits look alike.

2.1.3. Peer Entity authentication

 The third property we're concerned with is PEER ENTITY
 AUTHENTICATION.  What we mean by this is that we know that one of the
 endpoints in the communication is the one we intended.  Without peer
 entity authentication, it's very difficult to provide either
 confidentiality or data integrity.  For instance, if we receive a
 message from Alice, the property of data integrity doesn't do us much
 good unless we know that it was in fact sent by Alice and not the
 attacker.  Similarly, if we want to send a confidential message to
 Bob, it's not of much value to us if we're actually sending a
 confidential message to the attacker.

Rescorla & Korver Best Current Practice [Page 4] RFC 3552 Security Considerations Guidelines July 2003

 Note that peer entity authentication can be provided asymmetrically.
 When you call someone on the phone, you can be fairly certain that
 you have the right person -- or at least that you got a person who's
 actually at the phone number you called.  On the other hand, if they
 don't have caller ID, then the receiver of a phone call has no idea
 who's calling them.  Calling someone on the phone is an example of
 recipient authentication, since you know who the recipient of the
 call is, but they don't know anything about the sender.
 In messaging situations, you often wish to use peer entity
 authentication to establish the identity of the sender of a certain
 message.  In such contexts, this property is called DATA ORIGIN

2.2. Non-Repudiation

 A system that provides endpoint authentication allows one party to be
 certain of the identity of someone with whom he is communicating.
 When the system provides data integrity a receiver can be sure of
 both the sender's identity and that he is receiving the data that
 that sender meant to send.  However, he cannot necessarily
 demonstrate this fact to a third party.  The ability to make this
 demonstration is called NON-REPUDIATION.
 There are many situations in which non-repudiation is desirable.
 Consider the situation in which two parties have signed a contract
 which one party wishes to unilaterally abrogate.  He might simply
 claim that he had never signed it in the first place.  Non-
 repudiation prevents him from doing so, thus protecting the
 Unfortunately, non-repudiation can be very difficult to achieve in
 practice and naive approaches are generally inadequate.  Section 4.3
 describes some of the difficulties, which generally stem from the
 fact that the interests of the two parties are not aligned -- one
 party wishes to prove something that the other party wishes to deny.

2.3. Systems Security

 In general, systems security is concerned with protecting one's
 machines and data.  The intent is that machines should be used only
 by authorized users and for the purposes that the owners intend.
 Furthermore, they should be available for those purposes.  Attackers
 should not be able to deprive legitimate users of resources.

Rescorla & Korver Best Current Practice [Page 5] RFC 3552 Security Considerations Guidelines July 2003

2.3.1. Unauthorized Usage

 Most systems are not intended to be completely accessible to the
 public.  Rather, they are intended to be used only by certain
 authorized individuals.  Although many Internet services are
 available to all Internet users, even those servers generally offer a
 larger subset of services to specific users.  For instance, Web
 Servers often will serve data to any user, but restrict the ability
 to modify pages to specific users.  Such modifications by the general
 public would be UNAUTHORIZED USAGE.

2.3.2. Inappropriate Usage

 Being an authorized user does not mean that you have free run of the
 system.  As we said above, some activities are restricted to
 authorized users, some to specific users, and some activities are
 generally forbidden to all but administrators.  Moreover, even
 activities which are in general permitted might be forbidden in some
 cases.  For instance, users may be permitted to send email but
 forbidden from sending files above a certain size, or files which
 contain viruses.  These are examples of INAPPROPRIATE USAGE.

2.3.3. Denial of Service

 Recall that our third goal was that the system should be available to
 legitimate users.  A broad variety of attacks are possible which
 threaten such usage.  Such attacks are collectively referred to as
 DENIAL OF SERVICE attacks.  Denial of service attacks are often very
 easy to mount and difficult to stop.  Many such attacks are designed
 to consume machine resources, making it difficult or impossible to
 serve legitimate users.  Other attacks cause the target machine to
 crash, completely denying service to users.

3. The Internet Threat Model

 A THREAT MODEL describes the capabilities that an attacker is assumed
 to be able to deploy against a resource.  It should contain such
 information as the resources available to an attacker in terms of
 information, computing capability, and control of the system.  The
 purpose of a threat model is twofold.  First, we wish to identify the
 threats we are concerned with.  Second, we wish to rule some threats
 explicitly out of scope.  Nearly every security system is vulnerable
 to a sufficiently dedicated and resourceful attacker.
 The Internet environment has a fairly well understood threat model.
 In general, we assume that the end-systems engaging in a protocol
 exchange have not themselves been compromised.  Protecting against an
 attack when one of the end-systems has been compromised is

Rescorla & Korver Best Current Practice [Page 6] RFC 3552 Security Considerations Guidelines July 2003

 extraordinarily difficult.  It is, however, possible to design
 protocols which minimize the extent of the damage done under these
 By contrast, we assume that the attacker has nearly complete control
 of the communications channel over which the end-systems communicate.
 This means that the attacker can read any PDU (Protocol Data Unit) on
 the network and undetectably remove, change, or inject forged packets
 onto the wire.  This includes being able to generate packets that
 appear to be from a trusted machine.  Thus, even if the end-system
 with which you wish to communicate is itself secure, the Internet
 environment provides no assurance that packets which claim to be from
 that system in fact are.
 It's important to realize that the meaning of a PDU is different at
 different levels.  At the IP level, a PDU means an IP packet.  At the
 TCP level, it means a TCP segment.  At the application layer, it
 means some kind of application PDU.  For instance, at the level of
 email, it might either mean an RFC-822 message or a single SMTP
 command.  At the HTTP level, it might mean a request or response.

3.1. Limited Threat Models

 As we've said, a resourceful and dedicated attacker can control the
 entire communications channel.  However, a large number of attacks
 can be mounted by an attacker with fewer resources.  A number of
 currently known attacks can be mounted by an attacker with limited
 control of the network.  For instance, password sniffing attacks can
 be mounted by an attacker who can only read arbitrary packets.  This
 is generally referred to as a PASSIVE ATTACK [INTAUTH].
 By contrast, Morris' sequence number guessing attack [SEQNUM] can be
 mounted by an attacker who can write but not read arbitrary packets.
 Any attack which requires the attacker to write to the network is
 known as an ACTIVE ATTACK.
 Thus, a useful way of organizing attacks is to divide them based on
 the capabilities required to mount the attack.  The rest of this
 section describes these categories and provides some examples of each

3.2. Passive Attacks

 In a passive attack, the attacker reads packets off the network but
 does not write them.  The simplest way to mount such an attack is to
 simply be on the same LAN as the victim.  On most common LAN
 configurations, including Ethernet, 802.3, and FDDI, any machine on
 the wire can read all traffic destined for any other machine on the

Rescorla & Korver Best Current Practice [Page 7] RFC 3552 Security Considerations Guidelines July 2003

 same LAN.  Note that switching hubs make this sort of sniffing
 substantially more difficult, since traffic destined for a machine
 only goes to the network segment which that machine is on.
 Similarly, an attacker who has control of a host in the
 communications path between two victim machines is able to mount a
 passive attack on their communications.  It is also possible to
 compromise the routing infrastructure to specifically arrange that
 traffic passes through a compromised machine.  This might involve an
 active attack on the routing infrastructure to facilitate a passive
 attack on a victim machine.
 Wireless communications channels deserve special consideration,
 especially with the recent and growing popularity of wireless-based
 LANs, such as those using 802.11.  Since the data is simply broadcast
 on well known radio frequencies, an attacker simply needs to be able
 to receive those transmissions.  Such channels are especially
 vulnerable to passive attacks.  Although many such channels include
 cryptographic protection, it is often of such poor quality as to be
 nearly useless [WEP].
 In general, the goal of a passive attack is to obtain information
 which the sender and receiver would prefer to remain private.  This
 private information may include credentials useful in the electronic
 world and/or passwords or credentials useful in the outside world,
 such as confidential business information.

3.2.1. Confidentiality Violations

 The classic example of passive attack is sniffing some inherently
 private data off of the wire.  For instance, despite the wide
 availability of SSL, many credit card transactions still traverse the
 Internet in the clear.  An attacker could sniff such a message and
 recover the credit card number, which can then be used to make
 fraudulent transactions.  Moreover, confidential business information
 is routinely transmitted over the network in the clear in email.

3.2.2. Password Sniffing

 Another example of a passive attack is PASSWORD SNIFFING.  Password
 sniffing is directed towards obtaining unauthorized use of resources.
 Many protocols, including [TELNET], [POP], and [NNTP] use a shared
 password to authenticate the client to the server.  Frequently, this
 password is transmitted from the client to the server in the clear
 over the communications channel.  An attacker who can read this
 traffic can therefore capture the password and REPLAY it.  In other
 words, the attacker can initiate a connection to the server and pose
 as the client and login using the captured password.

Rescorla & Korver Best Current Practice [Page 8] RFC 3552 Security Considerations Guidelines July 2003

 Note that although the login phase of the attack is active, the
 actual password capture phase is passive.  Moreover, unless the
 server checks the originating address of connections, the login phase
 does not require any special control of the network.

3.2.3. Offline Cryptographic Attacks

 Many cryptographic protocols are subject to OFFLINE ATTACKS.  In such
 a protocol, the attacker recovers data which has been processed using
 the victim's secret key and then mounts a cryptanalytic attack on
 that key.  Passwords make a particularly vulnerable target because
 they are typically low entropy.  A number of popular password-based
 challenge response protocols are vulnerable to DICTIONARY ATTACK.
 The attacker captures a challenge-response pair and then proceeds to
 try entries from a list of common words (such as a dictionary file)
 until he finds a password that produces the right response.
 A similar such attack can be mounted on a local network when NIS is
 used.  The Unix password is crypted using a one-way function, but
 tools exist to break such crypted passwords [KLEIN].  When NIS is
 used, the crypted password is transmitted over the local network and
 an attacker can thus sniff the password and attack it.
 Historically, it has also been possible to exploit small operating
 system security holes to recover the password file using an active
 attack.  These holes can then be bootstrapped into an actual account
 by using the aforementioned offline password recovery techniques.
 Thus we combine a low-level active attack with an offline passive

3.3. Active Attacks

 When an attack involves writing data to the network, we refer to this
 as an ACTIVE ATTACK.  When IP is used without IPsec, there is no
 authentication for the sender address.  As a consequence, it's
 straightforward for an attacker to create a packet with a source
 address of his choosing.  We'll refer to this as a SPOOFING ATTACK.
 Under certain circumstances, such a packet may be screened out by the
 network.  For instance, many packet filtering firewalls screen out
 all packets with source addresses on the INTERNAL network that arrive
 on the EXTERNAL interface.  Note, however, that this provides no
 protection against an attacker who is inside the firewall.  In
 general, designers should assume that attackers can forge packets.

Rescorla & Korver Best Current Practice [Page 9] RFC 3552 Security Considerations Guidelines July 2003

 However, the ability to forge packets does not go hand in hand with
 the ability to receive arbitrary packets.  In fact, there are active
 attacks that involve being able to send forged packets but not
 receive the responses.  We'll refer to these as BLIND ATTACKS.
 Note that not all active attacks require forging addresses.  For
 instance, the TCP SYN denial of service attack [TCPSYN] can be
 mounted successfully without disguising the sender's address.
 However, it is common practice to disguise one's address in order to
 conceal one's identity if an attack is discovered.
 Each protocol is susceptible to specific active attacks, but
 experience shows that a number of common patterns of attack can be
 adapted to any given protocol.  The next sections describe a number
 of these patterns and give specific examples of them as applied to
 known protocols.

3.3.1. Replay Attacks

 In a REPLAY ATTACK, the attacker records a sequence of messages off
 of the wire and plays them back to the party which originally
 received them.  Note that the attacker does not need to be able to
 understand the messages.  He merely needs to capture and retransmit
 For example, consider the case where an S/MIME message is being used
 to request some service, such as a credit card purchase or a stock
 trade.  An attacker might wish to have the service executed twice, if
 only to inconvenience the victim.  He could capture the message and
 replay it, even though he can't read it, causing the transaction to
 be executed twice.

3.3.2. Message Insertion

 In a MESSAGE INSERTION attack, the attacker forges a message with
 some chosen set of properties and injects it into the network.  Often
 this message will have a forged source address in order to disguise
 the identity of the attacker.
 For example, a denial-of-service attack can be mounted by inserting a
 series of spurious TCP SYN packets directed towards the target host.
 The target host responds with its own SYN and allocates kernel data
 structures for the new connection.  The attacker never completes the
 3-way handshake, so the allocated connection endpoints just sit there
 taking up kernel memory.  Typical TCP stack implementations only

Rescorla & Korver Best Current Practice [Page 10] RFC 3552 Security Considerations Guidelines July 2003

 allow some limited number of connections in this "half-open" state
 and when this limit is reached, no more connections can be initiated,
 even from legitimate hosts.  Note that this attack is a blind attack,
 since the attacker does not need to process the victim's SYNs.

3.3.3. Message Deletion

 In a MESSAGE DELETION attack, the attacker removes a message from the
 wire.  Morris' sequence number guessing attack [SEQNUM] often
 requires a message deletion attack to be performed successfully.  In
 this blind attack, the host whose address is being forged will
 receive a spurious TCP SYN packet from the host being attacked.
 Receipt of this SYN packet generates a RST, which would tear the
 illegitimate connection down.  In order to prevent this host from
 sending a RST so that the attack can be carried out successfully,
 Morris describes flooding this host to create queue overflows such
 that the SYN packet is lost and thus never responded to.

3.3.4. Message Modification

 In a MESSAGE MODIFICATION attack, the attacker removes a message from
 the wire, modifies it, and reinjects it into the network.  This sort
 of attack is particularly useful if the attacker wants to send some
 of the data in the message but also wants to change some of it.
 Consider the case where the attacker wants to attack an order for
 goods placed over the Internet.  He doesn't have the victim's credit
 card number so he waits for the victim to place the order and then
 replaces the delivery address (and possibly the goods description)
 with his own.  Note that this particular attack is known as a CUT-
 AND-PASTE attack since the attacker cuts the credit card number out
 of the original message and pastes it into the new message.
 Another interesting example of a cut-and-paste attack is provided by
 [IPSPPROB].  If IPsec ESP is used without any MAC then it is possible
 for the attacker to read traffic encrypted for a victim on the same
 machine.  The attacker attaches an IP header corresponding to a port
 he controls onto the encrypted IP packet.  When the packet is
 received by the host it will automatically be decrypted and forwarded
 to the attacker's port.  Similar techniques can be used to mount a
 session hijacking attack.  Both of these attacks can be avoided by
 always using message authentication when you use encryption.  Note
 that this attack only works if (1) no MAC check is being used, since
 this attack generates damaged packets (2) a host-to-host SA is being
 used, since a user-to-user SA will result in an inconsistency between
 the port associated with the SA and the target port.  If the
 receiving machine is single-user than this attack is infeasible.

Rescorla & Korver Best Current Practice [Page 11] RFC 3552 Security Considerations Guidelines July 2003

3.3.5. Man-In-The-Middle

 A MAN-IN-THE-MIDDLE attack combines the above techniques in a special
 form: The attacker subverts the communication stream in order to pose
 as the sender to receiver and the receiver to the sender:
    What Alice and Bob think:
    Alice  <---------------------------------------------->  Bob
    What's happening:
    Alice  <---------------->  Attacker  <---------------->  Bob
 This differs fundamentally from the above forms of attack because it
 attacks the identity of the communicating parties, rather than the
 data stream itself.  Consequently, many techniques which provide
 integrity of the communications stream are insufficient to protect
 against man-in-the-middle attacks.
 Man-in-the-middle attacks are possible whenever a protocol lacks PEER
 ENTITY AUTHENTICATION.  For instance, if an attacker can hijack the
 client TCP connection during the TCP handshake (perhaps by responding
 to the client's SYN before the server does), then the attacker can
 open another connection to the server and begin a man-in-the-middle
 attack.  It is also trivial to mount man-in-the-middle attacks on
 local networks via ARP spoofing -- the attacker forges an ARP with
 the victim's IP address and his own MAC address.  Tools to mount this
 sort of attack are readily available.
 Note that it is only necessary to authenticate one side of the
 transaction in order to prevent man-in-the-middle attacks.  In such a
 situation the the peers can establish an association in which only
 one peer is authenticated.  In such a system, an attacker can
 initiate an association posing as the unauthenticated peer but cannot
 transmit or access data being sent on a legitimate connection.  This
 is an acceptable situation in contexts such as Web e-commerce where
 only the server needs to be authenticated (or the client is
 independently authenticated via some non-cryptographic mechanism such
 as a credit card number).

3.4. Topological Issues

 In practice, the assumption that it's equally easy for an attacker to
 read and generate all packets is false, since the Internet is not
 fully connected.  This has two primary implications.

Rescorla & Korver Best Current Practice [Page 12] RFC 3552 Security Considerations Guidelines July 2003

3.5. On-path versus off-path

 In order for a datagram to be transmitted from one host to another,
 it generally must traverse some set of intermediate links and
 gateways.  Such gateways are naturally able to read, modify, or
 remove any datagram transmitted along that path.  This makes it much
 easier to mount a wide variety of attacks if you are on-path.
 Off-path hosts can, of course, transmit arbitrary datagrams that
 appear to come from any hosts but cannot necessarily receive
 datagrams intended for other hosts.  Thus, if an attack depends on
 being able to receive data, off-path hosts must first subvert the
 topology in order to place themselves on-path.  This is by no means
 impossible but is not necessarily trivial.
 Applications protocol designers MUST NOT assume that all attackers
 will be off-path.  Where possible, protocols SHOULD be designed to
 resist attacks from attackers who have complete control of the
 network.  However, designers are expected to give more weight to
 attacks which can be mounted by off-path attackers as well as on-path

3.6. Link-local

 One specialized case of on-path is being on the same link.  In some
 situations, it's desirable to distinguish between hosts who are on
 the local network and those who are not.  The standard technique for
 this is verifying the IP TTL value [IP].  Since the TTL must be
 decremented by each forwarder, a protocol can demand that TTL be set
 to 255 and that all receivers verify the TTL.  A receiver then has
 some reason to believe that conforming packets are from the same
 link.  Note that this technique must be used with care in the
 presence of tunneling systems, since such systems may pass packets
 without decrementing TTL.

4. Common Issues

 Although each system's security requirements are unique, certain
 common requirements appear in a number of protocols.  Often, when
 naive protocol designers are faced with these requirements, they
 choose an obvious but insecure solution even though better solutions
 are available.  This section describes a number of issues seen in
 many protocols and the common pieces of security technology that may
 be useful in addressing them.

Rescorla & Korver Best Current Practice [Page 13] RFC 3552 Security Considerations Guidelines July 2003

4.1. User Authentication

 Essentially every system which wants to control access to its
 resources needs some way to authenticate users.  A nearly uncountable
 number of such mechanisms have been designed for this purpose.  The
 next several sections describe some of these techniques.

4.1.1. Username/Password

 The most common access control mechanism is simple USERNAME/PASSWORD
 The user provides a username and a reusable password to the host
 which he wishes to use.  This system is vulnerable to a simple
 passive attack where the attacker sniffs the password off the wire
 and then initiates a new session, presenting the password.  This
 threat can be mitigated by hosting the protocol over an encrypted
 connection such as TLS or IPSEC.  Unprotected (plaintext)
 username/password systems are not acceptable in IETF standards.

4.1.2. Challenge Response and One Time Passwords

 Systems which desire greater security than USERNAME/PASSWORD often
 employ either a ONE TIME PASSWORD [OTP] scheme or a CHALLENGE-
 RESPONSE.  In a one time password scheme, the user is provided with a
 list of passwords, which must be used in sequence, one time each.
 (Often these passwords are generated from some secret key so the user
 can simply compute the next password in the sequence.)  SecureID and
 DES Gold are variants of this scheme.  In a challenge-response
 scheme, the host and the user share some secret (which often is
 represented as a password).  In order to authenticate the user, the
 host presents the user with a (randomly generated) challenge.  The
 user computes some function based on the challenge and the secret and
 provides that to the host, which verifies it.  Often this computation
 is performed in a handheld device, such as a DES Gold card.
 Both types of scheme provide protection against replay attack, but
 often still vulnerable to an OFFLINE KEYSEARCH ATTACK (a form of
 passive attack): As previously mentioned, often the one-time password
 or response is computed from a shared secret.  If the attacker knows
 the function being used, he can simply try all possible shared
 secrets until he finds one that produces the right output.  This is
 made easier if the shared secret is a password, in which case he can
 mount a DICTIONARY ATTACK -- meaning that he tries a list of common
 words (or strings) rather than just random strings.
 These systems are also often vulnerable to an active attack.  Unless
 communication security is provided for the entire session, the
 attacker can simply wait until authentication has been performed and
 hijack the connection.

Rescorla & Korver Best Current Practice [Page 14] RFC 3552 Security Considerations Guidelines July 2003

4.1.3. Shared Keys

 CHALLENGE-RESPONSE type systems can be made secure against dictionary
 attack by using randomly generated shared keys instead of user-
 generated passwords.  If the keys are sufficiently large then
 keysearch attacks become impractical.  This approach works best when
 the keys are configured into the end nodes rather than memorized and
 typed in by users, since users have trouble remembering sufficiently
 long keys.
 Like password-based systems, shared key systems suffer from
 management problems.  Each pair of communicating parties must have
 their own agreed-upon key, which leads to there being a lot of keys.

4.1.4. Key Distribution Centers

 One approach to solving the large number of keys problem is to use an
 online "trusted third party" that mediates between the authenticating
 parties.  The trusted third party (generally called a a KEY
 DISTRIBUTION CENTER (KDC)) shares a symmetric key or password with
 each party in the system.  It first contacts the KDC which gives it a
 TICKET containing a randomly generated symmetric key encrypted under
 both peer's keys.  Since only the proper peers can decrypt the
 symmetric key the ticket can be used to establish a trusted
 association.  By far the most popular KDC system is Kerberos

4.1.5. Certificates

 A simple approach is to have all users have CERTIFICATES [PKIX] which
 they then use to authenticate in some protocol-specific way, as in
 [TLS] or [S/MIME].  A certificate is a signed credential binding an
 entity's identity to its public key.  The signer of a certificate is
 a CERTIFICATE AUTHORITY (CA), whose certificate may itself be signed
 by some superior CA.  In order for this system to work, trust in one
 or more CAs must be established in an out-of-band fashion.  Such CAs
 are referred to as TRUSTED ROOTS or ROOT CAS.  The primary obstacle
 to this approach in client-server type systems is that it requires
 clients to have certificates, which can be a deployment problem.

4.1.6. Some Uncommon Systems

 There are ways to do a better job than the schemes mentioned above,
 but they typically don't add much security unless communications
 security (at least message integrity) will be employed to secure the
 connection, because otherwise the attacker can merely hijack the
 connection after authentication has been performed.  A number of
 protocols ([EKE], [SPEKE], [SRP]) allow one to securely bootstrap a

Rescorla & Korver Best Current Practice [Page 15] RFC 3552 Security Considerations Guidelines July 2003

 user's password into a shared key which can be used as input to a
 cryptographic protocol.  One major obstacle to the deployment of
 these protocols has been that their Intellectual Property status is
 extremely unclear.  Similarly, the user can authenticate using public
 key certificates (e.g., S-HTTP client authentication).  Typically
 these methods are used as part of a more complete security protocol.

4.1.7. Host Authentication

 Host authentication presents a special problem.  Quite commonly, the
 addresses of services are presented using a DNS hostname, for
 instance as a URL [URL].  When requesting such a service, one has to
 ensure that the entity that one is talking to not only has a
 certificate but that that certificate corresponds to the expected
 identity of the server.  The important thing to have is a secure
 binding between the certificate and the expected hostname.
 For instance, it is usually not acceptable for the certificate to
 contain an identity in the form of an IP address if the request was
 for a given hostname.  This does not provide end-to-end security
 because the hostname-IP mapping is not secure unless secure name
 resolution [DNSSEC] is being used.  This is a particular problem when
 the hostname is presented at the application layer but the
 authentication is performed at some lower layer.

4.2. Generic Security Frameworks

 Providing security functionality in a protocol can be difficult.  In
 addition to the problem of choosing authentication and key
 establishment mechanisms, one needs to integrate it into a protocol.
 One response to this problem (embodied in IPsec and TLS) is to create
 a lower-level security protocol and then insist that new protocols be
 run over that protocol.  Another approach that has recently become
 popular is to design generic application layer security frameworks.
 The idea is that you design a protocol that allows you to negotiate
 various security mechanisms in a pluggable fashion.  Application
 protocol designers then arrange to carry the security protocol PDUs
 in their application protocol.  Examples of such frameworks include
 The generic framework approach has a number of problems.  First, it
 is highly susceptible to DOWNGRADE ATTACKS.  In a downgrade attack,
 an active attacker tampers with the negotiation in order to force the
 parties to negotiate weaker protection than they otherwise would.
 It's possible to include an integrity check after the negotiation and
 key establishment have both completed, but the strength of this
 integrity check is necessarily limited to the weakest common
 algorithm.  This problem exists with any negotiation approach, but

Rescorla & Korver Best Current Practice [Page 16] RFC 3552 Security Considerations Guidelines July 2003

 generic frameworks exacerbate it by encouraging the application
 protocol author to just specify the framework rather than think hard
 about the appropriate underlying mechanisms, particularly since the
 mechanisms can very widely in the degree of security offered.
 Another problem is that it's not always obvious how the various
 security features in the framework interact with the application
 layer protocol.  For instance, SASL can be used merely as an
 authentication framework -- in which case the SASL exchange occurs
 but the rest of the connection is unprotected, but can also negotiate
 traffic protection, such as via GSS, as a mechanism.  Knowing under
 what circumstances traffic protection is optional and which it is
 required requires thinking about the threat model.
 In general, authentication frameworks are most useful in situations
 where new protocols are being added to systems with pre-existing
 legacy authentication systems.  A framework allows new installations
 to provide better authentication while not forcing existing sites
 completely redo their legacy authentication systems.  When the
 security requirements of a system can be clearly identified and only
 a few forms of authentication are used, choosing a single security
 mechanism leads to greater simplicity and predictability.  In
 situations where a framework is to be used, designers SHOULD
 carefully examine the framework's options and specify only the
 mechanisms that are appropriate for their particular threat model.
 If a framework is necessary, designers SHOULD choose one of the
 established ones instead of designing their own.

4.3. Non-repudiation

 The naive approach to non-repudiation is simply to use public-key
 digital signatures over the content.  The party who wishes to be
 bound (the SIGNING PARTY) digitally signs the message in question.
 The counterparty (the RELYING PARTY) can later point to the digital
 signature as proof that the signing party at one point agreed to the
 disputed message.  Unfortunately, this approach is insufficient.
 The easiest way for the signing party to repudiate the message is by
 claiming that his private key has been compromised and that some
 attacker (though not necessarily the relying party) signed the
 disputed message.  In order to defend against this attack the relying
 party needs to demonstrate that the signing party's key had not been
 compromised at the time of the signature.  This requires substantial
 infrastructure, including archival storage of certificate revocation
 information and timestamp servers to establish the time that the
 message was signed.

Rescorla & Korver Best Current Practice [Page 17] RFC 3552 Security Considerations Guidelines July 2003

 Additionally, the relying party might attempt to trick the signing
 party into signing one message while thinking he's signing another.
 This problem is particularly severe when the relying party controls
 the infrastructure that the signing party uses for signing, such as
 in kiosk situations.  In many such situations the signing party's key
 is kept on a smartcard but the message to be signed is displayed by
 the relying party.
 All of these complications make non-repudiation a difficult service
 to deploy in practice.

4.4. Authorization vs. Authentication

 AUTHORIZATION is the process by which one determines whether an
 authenticated party has permission to access a particular resource or
 service.  Although tightly bound, it is important to realize that
 authentication and authorization are two separate mechanisms.
 Perhaps because of this tight coupling, authentication is sometimes
 mistakenly thought to imply authorization.  Authentication simply
 identifies a party, authorization defines whether they can perform a
 certain action.
 Authorization necessarily relies on authentication, but
 authentication alone does not imply authorization.  Rather, before
 granting permission to perform an action, the authorization mechanism
 must be consulted to determine whether that action is permitted.

4.4.1. Access Control Lists

 One common form of authorization mechanism is an access control list
 (ACL), which lists users that are permitted access to a resource.
 Since assigning individual authorization permissions to each resource
 is tedious, resources are often hierarchically arranged so that the
 parent resource's ACL is inherited by child resources.  This allows
 administrators to set top level policies and override them when

4.4.2. Certificate Based Systems

 While the distinction between authentication and authorization is
 intuitive when using simple authentication mechanisms such as
 username and password (i.e., everyone understands the difference
 between the administrator account and a user account), with more
 complex authentication mechanisms the distinction is sometimes lost.
 With certificates, for instance, presenting a valid signature does
 not imply authorization.  The signature must be backed by a
 certificate chain that contains a trusted root, and that root must be

Rescorla & Korver Best Current Practice [Page 18] RFC 3552 Security Considerations Guidelines July 2003

 trusted in the given context.  For instance, users who possess
 certificates issued by the Acme MIS CA may have different web access
 privileges than users who possess certificates issued by the Acme
 Accounting CA, even though both of these CAs are "trusted" by the
 Acme web server.
 Mechanisms for enforcing these more complicated properties have not
 yet been completely explored.  One approach is simply to attach
 policies to ACLs describing what sorts of certificates are trusted.
 Another approach is to carry that information with the certificate,
 either as a certificate extension/attribute [PKIX, SPKI] or as a
 separate "Attribute Certificate".

4.5. Providing Traffic Security

 Securely designed protocols should provide some mechanism for
 securing (meaning integrity protecting, authenticating, and possibly
 encrypting) all sensitive traffic.  One approach is to secure the
 protocol itself, as in [DNSSEC], [S/MIME] or [S-HTTP].  Although this
 provides security which is most fitted to the protocol, it also
 requires considerable effort to get right.
 Many protocols can be adequately secured using one of the available
 channel security systems.  We'll discuss the two most common, IPsec
 [AH, ESP] and [TLS].

4.5.1. IPsec

 The IPsec protocols (specifically, AH and ESP) can provide
 transmission security for all traffic between two hosts.  The IPsec
 protocols support varying granularities of user identification,
 including for example "IP Subnet", "IP Address", "Fully Qualified
 Domain Name", and individual user ("Mailbox name").  These varying
 levels of identification are employed as inputs to access control
 facilities that are an intrinsic part of IPsec.  However, a given
 IPsec implementation might not support all identity types.  In
 particular, security gateways may not provide user-to-user
 authentication or have mechanisms to provide that authentication
 information to applications.
 When AH or ESP is used, the application programmer might not need to
 do anything (if AH or ESP has been enabled system-wide) or might need
 to make specific software changes (e.g., adding specific setsockopt()
 calls) -- depending on the AH or ESP implementation being used.
 Unfortunately, APIs for controlling IPsec implementations are not yet

Rescorla & Korver Best Current Practice [Page 19] RFC 3552 Security Considerations Guidelines July 2003

 The primary obstacle to using IPsec to secure other protocols is
 deployment.  The major use of IPsec at present is for VPN
 applications, especially for remote network access.  Without
 extremely tight coordination between security administrators and
 application developers, VPN usage is not well suited to providing
 security services for individual applications since it is difficult
 for such applications to determine what security services have in
 fact been provided.
 IPsec deployment in host-to-host environments has been slow.  Unlike
 application security systems such as TLS, adding IPsec to a non-IPsec
 system generally involves changing the operating system, either by
 modifying with the kernel or installing new drivers.  This is a
 substantially greater undertaking than simply installing a new
 application.  However, recent versions of a number of commodity
 operating systems include IPsec stacks, so deployment is becoming
 In environments where IPsec is sure to be available, it represents a
 viable option for protecting application communications traffic.  If
 the traffic to be protected is UDP, IPsec and application-specific
 object security are the only options.  However, designers MUST NOT
 assume that IPsec will be available.  A security policy for a generic
 application layer protocol SHOULD NOT simply state that IPsec must be
 used, unless there is some reason to believe that IPsec will be
 available in the intended deployment environment.  In environments
 where IPsec may not be available and the traffic is solely TCP, TLS
 is the method of choice, since the application developer can easily
 ensure its presence by including a TLS implementation in his package.
 In the special-case of IPv6, both AH and ESP are mandatory to
 implement.  Hence, it is reasonable to assume that AH/ESP are already
 available for IPv6-only protocols or IPv6-only deployments.  However,
 automatic key management (IKE) is not required to implement so
 protocol designers SHOULD not assume it will be present.  [USEIPSEC]
 provides quite a bit of guidance on when IPsec is a good choice.

4.5.2. SSL/TLS

 Currently, the most common approach is to use SSL or its successor
 TLS.  They provide channel security for a TCP connection at the
 application level.  That is, they run over TCP.  SSL implementations
 typically provide a Berkeley Sockets-like interface for easy
 programming.  The primary issue when designing a protocol solution
 around TLS is to differentiate between connections protected using
 TLS and those which are not.

Rescorla & Korver Best Current Practice [Page 20] RFC 3552 Security Considerations Guidelines July 2003

 The two primary approaches used have a separate well-known port for
 TLS connections (e.g., the HTTP over TLS port is 443) [HTTPTLS] or to
 have a mechanism for negotiating upward from the base protocol to TLS
 as in [UPGRADE] or [STARTTLS].  When an upward negotiation strategy
 is used, care must be taken to ensure that an attacker can not force
 a clear connection when both parties wish to use TLS.
 Note that TLS depends upon a reliable protocol such as TCP or SCTP.
 This produces two notable difficulties.  First, it cannot be used to
 secure datagram protocols that use UDP.  Second, TLS is susceptible
 to IP layer attacks that IPsec is not.  Typically, these attacks take
 some form of denial of service or connection assassination.  For
 instance, an attacker might forge a TCP RST to shut down SSL
 connections.  TLS has mechanisms to detect truncation attacks but
 these merely allow the victim to know he is being attacked and do not
 provide connection survivability in the face of such attacks.  By
 contrast, if IPsec were being used, such a forged RST could be
 rejected without affecting the TCP connection.  If forged RSTs or
 other such attacks on the TCP connection are a concern, then AH/ESP
 or the TCP MD5 option [TCPMD5] are the preferred choices. Virtual Hosts

 If the "separate ports" approach to TLS is used, then TLS will be
 negotiated before any application-layer traffic is sent.  This can
 cause a problem with protocols that use virtual hosts, such as
 [HTTP], since the server does not know which certificate to offer the
 client during the TLS handshake.  The TLS hostname extension [TLSEXT]
 can be used to solve this problem, although it is too new to have
 seen wide deployment. Remote Authentication and TLS

 One difficulty with using TLS is that the server is authenticated via
 a certificate.  This can be inconvenient in environments where
 previously the only form of authentication was a password shared
 between client and server.  It's tempting to use TLS without an
 authenticated server (i.e., with anonymous DH or a self-signed RSA
 certificate) and then authenticate via some challenge-response
 mechanism such as SASL with CRAM-MD5.
 Unfortunately, this composition of SASL and TLS is less strong than
 one would expect.  It's easy for an active attacker to hijack this
 connection.  The client man-in-the-middles the SSL connection
 (remember we're not authenticating the server, which is what
 ordinarily prevents this attack) and then simply proxies the SASL
 handshake.  From then on, it's as if the connection were in the

Rescorla & Korver Best Current Practice [Page 21] RFC 3552 Security Considerations Guidelines July 2003

 clear, at least as far as that attacker is concerned.  In order to
 prevent this attack, the client needs to verify the server's
 However, if the server is authenticated, challenge-response becomes
 less desirable.  If you already have a hardened channel then simple
 passwords are fine.  In fact, they're arguably superior to
 challenge-response since they do not require that the password be
 stored in the clear on the server.  Thus, compromise of the key file
 with challenge-response systems is more serious than if simple
 passwords were used.
 Note that if the client has a certificate than SSL-based client
 authentication can be used.  To make this easier, SASL provides the
 EXTERNAL mechanism, whereby the SASL client can tell the server
 "examine the outer channel for my identity".  Obviously, this is not
 subject to the layering attacks described above.

4.5.3. Remote Login

 In some special cases it may be worth providing channel-level
 security directly in the application rather than using IPSEC or
 SSL/TLS.  One such case is remote terminal security.  Characters are
 typically delivered from client to server one character at a time.
 Since SSL/TLS and AH/ESP authenticate and encrypt every packet, this
 can mean a data expansion of 20-fold.  The telnet encryption option
 [ENCOPT] prevents this expansion by foregoing message integrity.
 When using remote terminal service, it's often desirable to securely
 perform other sorts of communications services.  In addition to
 providing remote login, SSH [SSH] also provides secure port
 forwarding for arbitrary TCP ports, thus allowing users run arbitrary
 TCP-based applications over the SSH channel.  Note that SSH Port
 Forwarding can be security issue if it is used improperly to
 circumvent firewall and improperly expose insecure internal
 applications to the outside world.

4.6. Denial of Service Attacks and Countermeasures

 Denial of service attacks are all too frequently viewed as an fact of
 life.  One problem is that an attacker can often choose from one of
 many denial of service attacks to inflict upon a victim, and because
 most of these attacks cannot be thwarted, common wisdom frequently
 assumes that there is no point protecting against one kind of denial
 of service attack when there are many other denial of service attacks
 that are possible but that cannot be prevented.

Rescorla & Korver Best Current Practice [Page 22] RFC 3552 Security Considerations Guidelines July 2003

 However, not all denial of service attacks are equal and more
 importantly, it is possible to design protocols so that denial of
 service attacks are made more difficult, if not impractical.  Recent
 SYN flood attacks [TCPSYN] demonstrate both of these properties: SYN
 flood attacks are so easy, anonymous, and effective that they are
 more attractive to attackers than other attacks; and because the
 design of TCP enables this attack.
 Because complete DoS protection is so difficult, security against DoS
 must be dealt with pragmatically.  In particular, some attacks which
 would be desirable to defend against cannot be defended against
 economically.  The goal should be to manage risk by defending against
 attacks with sufficiently high ratios of severity to cost of defense.
 Both severity of attack and cost of defense change as technology
 changes and therefore so does the set of attacks which should be
 defended against.
 Authors of internet standards MUST describe which denial of service
 attacks their protocol is susceptible to.  This description MUST
 include the reasons it was either unreasonable or out of scope to
 attempt to avoid these denial of service attacks.

4.6.1. Blind Denial of Service

 BLIND denial of service attacks are particularly pernicious.  With a
 blind attack the attacker has a significant advantage.  If the
 attacker must be able to receive traffic from the victim, then he
 must either subvert the routing fabric or use his own IP address.
 Either provides an opportunity for the victim to track the attacker
 and/or filter out his traffic.  With a blind attack the attacker can
 use forged IP addresses, making it extremely difficult for the victim
 to filter out his packets.  The TCP SYN flood attack is an example of
 a blind attack.  Designers should make every attempt possible to
 prevent blind denial of service attacks.

4.6.2. Distributed Denial of Service

 Even more dangerous are DISTRIBUTED denial of service attacks (DDoS)
 [DDOS].  In a DDoS the attacker arranges for a number of machines to
 attack the target machine simultaneously.  Usually this is
 accomplished by infecting a large number of machines with a program
 that allows remote initiation of attacks.  The machines actually
 performing the attack are called ZOMBIEs and are likely owned by
 unsuspecting third parties in an entirely different location from the
 true attacker.  DDoS attacks can be very hard to counter because the
 zombies often appear to be making legitimate protocol requests and

Rescorla & Korver Best Current Practice [Page 23] RFC 3552 Security Considerations Guidelines July 2003

 simply crowd out the real users.  DDoS attacks can be difficult to
 thwart, but protocol designers are expected to be cognizant of these
 forms of attack while designing protocols.

4.6.3. Avoiding Denial of Service

 There are two common approaches to making denial of service attacks
 more difficult: Make your attacker do more work than you do

 If an attacker consumes more of his resources than yours when
 launching an attack, attackers with fewer resources than you will be
 unable to launch effective attacks.  One common technique is to
 require the attacker perform a time-intensive operation, such as a
 cryptographic operation.  Note that an attacker can still mount a
 denial of service attack if he can muster substantially sufficient
 CPU power.  For instance, this technique would not stop the
 distributed attacks described in [TCPSYN]. Make your attacker prove they can receive data from you

 A blind attack can be subverted by forcing the attacker to prove that
 they can can receive data from the victim.  A common technique is to
 require that the attacker reply using information that was gained
 earlier in the message exchange.  If this countermeasure is used, the
 attacker must either use his own address (making him easy to track)
 or to forge an address which will be routed back along a path that
 traverses the host from which the attack is being launched.
 Hosts on small subnets are thus useless to the attacker (at least in
 the context of a spoofing attack) because the attack can be traced
 back to a subnet (which should be sufficient for locating the
 attacker) so that anti-attack measures can be put into place (for
 instance, a boundary router can be configured to drop all traffic
 from that subnet).  A common technique is to require that the
 attacker reply using information that was gained earlier in the
 message exchange.

4.6.4. Example: TCP SYN Floods

 TCP/IP is vulnerable to SYN flood attacks (which are described in
 section 3.3.2) because of the design of the 3-way handshake.  First,
 an attacker can force a victim to consume significant resources (in
 this case, memory) by sending a single packet.  Second, because the
 attacker can perform this action without ever having received data
 from the victim, the attack can be performed anonymously (and
 therefore using a large number of forged source addresses).

Rescorla & Korver Best Current Practice [Page 24] RFC 3552 Security Considerations Guidelines July 2003

4.6.5. Example: Photuris

 [PHOTURIS] specifies an anti-clogging mechanism that prevents attacks
 on Photuris that resemble the SYN flood attack.  Photuris employs a
 time-variant secret to generate a "cookie" which is returned to the
 attacker.  This cookie must be returned in subsequent messages for
 the exchange to progress.  The interesting feature is that this
 cookie can be regenerated by the victim later in the exchange, and
 thus no state need be retained by the victim until after the attacker
 has proven that he can receive packets from the victim.

4.7. Object vs. Channel Security

 It's useful to make the conceptual distinction between object
 security and channel security.  Object security refers to security
 measures which apply to entire data objects.  Channel security
 measures provide a secure channel over which objects may be carried
 transparently but the channel has no special knowledge about object
 Consider the case of an email message.  When it's carried over an
 IPSEC or TLS secured connection, the message is protected during
 transmission.  However, it is unprotected in the receiver's mailbox,
 and in intermediate spool files along the way.  Moreover, since mail
 servers generally run as a daemon, not a user, authentication of
 messages generally merely means authentication of the daemon not the
 user.  Finally, since mail transport is hop-by-hop, even if the user
 authenticates to the first hop relay the authentication can't be
 safely verified by the receiver.
 By contrast, when an email message is protected with S/MIME or
 OpenPGP, the entire message is encrypted and integrity protected
 until it is examined and decrypted by the recipient.  It also
 provides strong authentication of the actual sender, as opposed to
 the machine the message came from.  This is object security.
 Moreover, the receiver can prove the signed message's authenticity to
 a third party.
 Note that the difference between object and channel security is a
 matter of perspective.  Object security at one layer of the protocol
 stack often looks like channel security at the next layer up.  So,
 from the perspective of the IP layer, each packet looks like an
 individually secured object.  But from the perspective of a web
 client, IPSEC just provides a secure channel.
 The distinction isn't always clear-cut.  For example, S-HTTP provides
 object level security for a single HTTP transaction, but a web page
 typically consists of multiple HTTP transactions (the base page and

Rescorla & Korver Best Current Practice [Page 25] RFC 3552 Security Considerations Guidelines July 2003

 numerous inline images).  Thus, from the perspective of the total web
 page, this looks rather more like channel security.  Object security
 for a web page would consist of security for the transitive closure
 of the page and all its embedded content as a single unit.

4.8. Firewalls and Network Topology

 It's common security practice in modern networks to partition the
 network into external and internal networks using a firewall.  The
 internal network is then assumed to be secure and only limited
 security measures are used there.  The internal portion of such a
 network is often called a WALLED GARDEN.
 Internet protocol designers cannot safely assume that their protocols
 will be deployed in such an environment, for three reasons.  First,
 protocols which were originally designed to be deployed in closed
 environments often are later deployed on the Internet, thus creating
 serious vulnerabilities.
 Second, networks which appear to be topologically disconnected may
 not be.  One reason may be that the network has been reconfigured to
 allow access by the outside world.  Moreover, firewalls are
 increasingly passing generic application layer protocols such as
 [SOAP] or [HTTP].  Network protocols which are based on these generic
 protocols cannot in general assume that a firewall will protect them.
 Finally, one of the most serious security threats to systems is from
 insiders, not outsiders.  Since insiders by definition have access to
 the internal network, topological protections such as firewalls will
 not protect them.

5. Writing Security Considerations Sections

 While it is not a requirement that any given protocol or system be
 immune to all forms of attack, it is still necessary for authors to
 consider as many forms as possible.  Part of the purpose of the
 Security Considerations section is to explain what attacks are out of
 scope and what countermeasures can be applied to defend against them.
 There should be a clear description of the kinds of threats on the
 described protocol or technology.  This should be approached as an
 effort to perform "due diligence" in describing all known or
 foreseeable risks and threats to potential implementers and users.

Rescorla & Korver Best Current Practice [Page 26] RFC 3552 Security Considerations Guidelines July 2003

 Authors MUST describe
    1.   which attacks are out of scope (and why!)
    2.   which attacks are in-scope
    2.1  and the protocol is susceptible to
    2.2  and the protocol protects against
 At least the following forms of attack MUST be considered:
 eavesdropping, replay, message insertion, deletion, modification, and
 man-in-the-middle.  Potential denial of service attacks MUST be
 identified as well.  If the protocol incorporates cryptographic
 protection mechanisms, it should be clearly indicated which portions
 of the data are protected and what the protections are (i.e.,
 integrity only, confidentiality, and/or endpoint authentication,
 etc.).  Some indication should also be given to what sorts of attacks
 the cryptographic protection is susceptible.  Data which should be
 held secret (keying material, random seeds, etc.) should be clearly
 If the technology involves authentication, particularly user-host
 authentication, the security of the authentication method MUST be
 clearly specified.  That is, authors MUST document the assumptions
 that the security of this authentication method is predicated upon.
 For instance, in the case of the UNIX username/password login method,
 a statement to the effect of:
    Authentication in the system is secure only to the extent that it
    is difficult to guess or obtain a ASCII password that is a maximum
    of 8 characters long.  These passwords can be obtained by sniffing
    telnet sessions or by running the 'crack' program using the
    contents of the /etc/passwd file.  Attempts to protect against
    on-line password guessing by (1) disconnecting after several
    unsuccessful login attempts and (2) waiting between successive
    password prompts is effective only to the extent that attackers
    are impatient.
    Because the /etc/passwd file maps usernames to user ids, groups,
    etc. it must be world readable.  In order to permit this usage but
    make running crack more difficult, the file is often split into
    /etc/passwd and a 'shadow' password file.  The shadow file is not
    world readable and contains the encrypted password.  The regular
    /etc/passwd file contains a dummy password in its place.
 It is insufficient to simply state that one's protocol should be run
 over some lower layer security protocol.  If a system relies upon
 lower layer security services for security, the protections those

Rescorla & Korver Best Current Practice [Page 27] RFC 3552 Security Considerations Guidelines July 2003

 services are expected to provide MUST be clearly specified.  In
 addition, the resultant properties of the combined system need to be
 Note: In general, the IESG will not approve standards track protocols
 which do not provide for strong authentication, either internal to
 the protocol or through tight binding to a lower layer security
 The threat environment addressed by the Security Considerations
 section MUST at a minimum include deployment across the global
 Internet across multiple administrative boundaries without assuming
 that firewalls are in place, even if only to provide justification
 for why such consideration is out of scope for the protocol.  It is
 not acceptable to only discuss threats applicable to LANs and ignore
 the broader threat environment.  All IETF standards-track protocols
 are considered likely to have deployment in the global Internet.  In
 some cases, there might be an Applicability Statement discouraging
 use of a technology or protocol in a particular environment.
 Nonetheless, the security issues of broader deployment should be
 discussed in the document.
 There should be a clear description of the residual risk to the user
 or operator of that protocol after threat mitigation has been
 deployed.  Such risks might arise from compromise in a related
 protocol (e.g., IPsec is useless if key management has been
 compromised), from incorrect implementation, compromise of the
 security technology used for risk reduction (e.g., a cipher with a
 40-bit key), or there might be risks that are not addressed by the
 protocol specification (e.g., denial of service attacks on an
 underlying link protocol).  Particular care should be taken in
 situations where the compromise of a single system would compromise
 an entire protocol.  For instance, in general protocol designers
 assume that end-systems are inviolate and don't worry about physical
 attack.  However, in cases (such as a certificate authority) where
 compromise of a single system could lead to widespread compromises,
 it is appropriate to consider systems and physical security as well.
 There should also be some discussion of potential security risks
 arising from potential misapplications of the protocol or technology
 described in the RFC.  This might be coupled with an Applicability
 Statement for that RFC.

6. Examples

 This section consists of some example security considerations
 sections, intended to give the reader a flavor of what's intended by
 this document.

Rescorla & Korver Best Current Practice [Page 28] RFC 3552 Security Considerations Guidelines July 2003

 The first example is a 'retrospective' example, applying the criteria
 of this document to an existing widely deployed protocol, SMTP.  The
 second example is a good security considerations section clipped from
 a current protocol.

6.1. SMTP

 When RFC 821 was written, Security Considerations sections were not
 required in RFCs, and none is contained in that document.  [RFC 2821]
 updated RFC 821 and added a detailed security considerations section.
 We reproduce here the Security Considerations section from that
 document (with new section numbers).  Our comments are indented and
 prefaced with 'NOTE:'.  We also add a number of new sections to cover
 topics we consider important.  Those sections are marked with [NEW]
 in the section header.

6.1.1. Security Considerations Mail Security and Spoofing

 SMTP mail is inherently insecure in that it is feasible for even
 fairly casual users to negotiate directly with receiving and relaying
 SMTP servers and create messages that will trick a naive recipient
 into believing that they came from somewhere else.  Constructing such
 a message so that the "spoofed" behavior cannot be detected by an
 expert is somewhat more difficult, but not sufficiently so as to be a
 deterrent to someone who is determined and knowledgeable.
 Consequently, as knowledge of Internet mail increases, so does the
 knowledge that SMTP mail inherently cannot be authenticated, or
 integrity checks provided, at the transport level.  Real mail
 security lies only in end-to-end methods involving the message
 bodies, such as those which use digital signatures (see [14] and,
 e.g., PGP [4] or S/MIME [31]).
    NOTE: One bad approach to sender authentication is [IDENT] in
    which the receiving mail server contacts the alleged sender and
    asks for the username of the sender.  This is a bad idea for a
    number of reasons, including but not limited to relaying, TCP
    connection hijacking, and simple lying by the origin server.
    Aside from the fact that IDENT is of low security value, use of
    IDENT by receiving sites can lead to operational problems.  Many
    sending sites blackhole IDENT requests, thus causing mail to be
    held until the receiving server's IDENT request times out.
 Various protocol extensions and configuration options that provide
 authentication at the transport level (e.g., from an SMTP client to
 an SMTP server) improve somewhat on the traditional situation
 described above.  However, unless they are accompanied by careful

Rescorla & Korver Best Current Practice [Page 29] RFC 3552 Security Considerations Guidelines July 2003

 handoffs of responsibility in a carefully-designed trust environment,
 they remain inherently weaker than end-to-end mechanisms which use
 digitally signed messages rather than depending on the integrity of
 the transport system.
 Efforts to make it more difficult for users to set envelope return
 path and header "From" fields to point to valid addresses other than
 their own are largely misguided: they frustrate legitimate
 applications in which mail is sent by one user on behalf of another
 or in which error (or normal) replies should be directed to a special
 address.  (Systems that provide convenient ways for users to alter
 these fields on a per-message basis should attempt to establish a
 primary and permanent mailbox address for the user so that Sender
 fields within the message data can be generated sensibly.)
 This specification does not further address the authentication issues
 associated with SMTP other than to advocate that useful functionality
 not be disabled in the hope of providing some small margin of
 protection against an ignorant user who is trying to fake mail.
    NOTE: We have added additional material on communications security
    and SMTP in Section 6.1.2 In a final specification, the above text
    would be edited somewhat to reflect that fact. Blind Copies

 Addresses that do not appear in the message headers may appear in the
 RCPT commands to an SMTP server for a number of reasons.  The two
 most common involve the use of a mailing address as a "list exploder"
 (a single address that resolves into multiple addresses) and the
 appearance of "blind copies".  Especially when more than one RCPT
 command is present, and in order to avoid defeating some of the
 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy
 the full set of RCPT command arguments into the headers, either as
 part of trace headers or as informational or private-extension
 headers.  Since this rule is often violated in practice, and cannot
 be enforced, sending SMTP systems that are aware of "bcc" use MAY
 find it helpful to send each blind copy as a separate message
 transaction containing only a single RCPT command.
 There is no inherent relationship between either "reverse" (from
 MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP
 transaction ("envelope") and the addresses in the headers.  Receiving
 systems SHOULD NOT attempt to deduce such relationships and use them

Rescorla & Korver Best Current Practice [Page 30] RFC 3552 Security Considerations Guidelines July 2003

 to alter the headers of the message for delivery.  The popular
 "Apparently-to" header is a violation of this principle as well as a
 common source of unintended information disclosure and SHOULD NOT be
 used. VRFY, EXPN, and Security

 As discussed in section 3.5, individual sites may want to disable
 either or both of VRFY or EXPN for security reasons.  As a corollary
 to the above, implementations that permit this MUST NOT appear to
 have verified addresses that are not, in fact, verified.  If a site
 disables these commands for security reasons, the SMTP server MUST
 return a 252 response, rather than a code that could be confused with
 successful or unsuccessful verification.
 Returning a 250 reply code with the address listed in the VRFY
 command after having checked it only for syntax violates this rule.
 Of course, an implementation that "supports" VRFY by always returning
 550 whether or not the address is valid is equally not in
 Within the last few years, the contents of mailing lists have become
 popular as an address information source for so-called "spammers."
 The use of EXPN to "harvest" addresses has increased as list
 administrators have installed protections against inappropriate uses
 of the lists themselves.  Implementations SHOULD still provide
 support for EXPN, but sites SHOULD carefully evaluate the tradeoffs.
 As authentication mechanisms are introduced into SMTP, some sites may
 choose to make EXPN available only to authenticated requesters.
    NOTE: It's not clear that disabling VRFY adds much protection,
    since it's often possible to discover whether an address is valid
    using RCPT TO. Information Disclosure in Announcements

 There has been an ongoing debate about the tradeoffs between the
 debugging advantages of announcing server type and version (and,
 sometimes, even server domain name) in the greeting response or in
 response to the HELP command and the disadvantages of exposing
 information that might be useful in a potential hostile attack.  The
 utility of the debugging information is beyond doubt.  Those who
 argue for making it available point out that it is far better to
 actually secure an SMTP server rather than hope that trying to
 conceal known vulnerabilities by hiding the server's precise identity
 will provide more protection.  Sites are encouraged to evaluate the

Rescorla & Korver Best Current Practice [Page 31] RFC 3552 Security Considerations Guidelines July 2003

 tradeoff with that issue in mind; implementations are strongly
 encouraged to minimally provide for making type and version
 information available in some way to other network hosts. Information Disclosure in Trace Fields

 In some circumstances, such as when mail originates from within a LAN
 whose hosts are not directly on the public Internet, trace
 ("Received") fields produced in conformance with this specification
 may disclose host names and similar information that would not
 normally be available.  This ordinarily does not pose a problem, but
 sites with special concerns about name disclosure should be aware of
 it.  Also, the optional FOR clause should be supplied with caution or
 not at all when multiple recipients are involved lest it
 inadvertently disclose the identities of "blind copy" recipients to
 others. Information Disclosure in Message Forwarding

 As discussed in section 3.4, use of the 251 or 551 reply codes to
 identify the replacement address associated with a mailbox may
 inadvertently disclose sensitive information.  Sites that are
 concerned about those issues should ensure that they select and
 configure servers appropriately. Scope of Operation of SMTP Servers

 It is a well-established principle that an SMTP server may refuse to
 accept mail for any operational or technical reason that makes sense
 to the site providing the server.  However, cooperation among sites
 and installations makes the Internet possible.  If sites take
 excessive advantage of the right to reject traffic, the ubiquity of
 email availability (one of the strengths of the Internet) will be
 threatened; considerable care should be taken and balance maintained
 if a site decides to be selective about the traffic it will accept
 and process.
 In recent years, use of the relay function through arbitrary sites
 has been used as part of hostile efforts to hide the actual origins
 of mail.  Some sites have decided to limit the use of the relay
 function to known or identifiable sources, and implementations SHOULD
 provide the capability to perform this type of filtering.  When mail
 is rejected for these or other policy reasons, a 550 code SHOULD be
 used in response to EHLO, MAIL, or RCPT as appropriate.

Rescorla & Korver Best Current Practice [Page 32] RFC 3552 Security Considerations Guidelines July 2003 Inappropriate Usage [NEW]

 SMTP itself provides no protection is provided against unsolicited
 commercial mass e-mail (aka spam).  It is extremely difficult to tell
 a priori whether a given message is spam or not.  From a protocol
 perspective, spam is indistinguishable from other e-mail -- the
 distinction is almost entirely social and often quite subtle.  (For
 instance, is a message from a merchant from whom you've purchased
 items before advertising similar items spam?) SMTP spam-suppression
 mechanisms are generally limited to identifying known spam senders
 and either refusing to service them or target them for
 punishment/disconnection.  [RFC-2505] provides extensive guidance on
 making SMTP servers spam-resistant.  We provide a brief discussion of
 the topic here.
 The primary tool for refusal to service spammers is the blacklist.
 Some authority such as [MAPS] collects and publishes a list of known
 spammers.  Individual SMTP servers then block the blacklisted
 offenders (generally by IP address).
 In order to avoid being blacklisted or otherwise identified, spammers
 often attempt to obscure their identity, either simply by sending a
 false SMTP identity or by forwarding their mail through an Open Relay
 -- an SMTP server which will perform mail relaying for any sender.
 As a consequence, there are now blacklists [ORBS] of open relays as
 well. Closed Relaying [NEW]

 To avoid being used for spam forwarding, many SMTP servers operate as
 closed relays, providing relaying service only for clients who they
 can identify.  Such relays should generally insist that senders
 advertise a sending address consistent with their known identity.  If
 the relay is providing service for an identifiable network (such as a
 corporate network or an ISP's network) then it is sufficient to block
 all other IP addresses).  In other cases, explicit authentication
 must be used.  The two standard choices for this are TLS [STARTTLS]
 and SASL [SASLSMTP]. Endpoints [NEW]

 Realistically, SMTP endpoints cannot refuse to deny service to
 unauthenticated senders.  Since the vast majority of senders are
 unauthenticated, this would break Internet mail interoperability.
 The exception to this is when the endpoint server should only be

Rescorla & Korver Best Current Practice [Page 33] RFC 3552 Security Considerations Guidelines July 2003

 receiving mail from some other server which can itself receive
 unauthenticated messages.  For instance, a company might operate a
 public gateway but configure its internal servers to only talk to the

6.1.2. Communications security issues [NEW]

 SMTP itself provides no communications security, and therefore a
 large number of attacks are possible.  A passive attack is sufficient
 to recover the text of messages transmitted with SMTP.  No endpoint
 authentication is provided by the protocol.  Sender spoofing is
 trivial, and therefore forging email messages is trivial.  Some
 implementations do add header lines with hostnames derived through
 reverse name resolution (which is only secure to the extent that it
 is difficult to spoof DNS -- not very), although these header lines
 are normally not displayed to users.  Receiver spoofing is also
 fairly straight-forward, either using TCP connection hijacking or DNS
 spoofing.  Moreover, since email messages often pass through SMTP
 gateways, all intermediate gateways must be trusted, a condition
 nearly impossible on the global Internet.
 Several approaches are available for alleviating these threats.  In
 order of increasingly high level in the protocol stack, we have:
    SMTP over IPSEC

 An SMTP connection run over IPSEC can provide confidentiality for the
 message between the sender and the first hop SMTP gateway, or between
 any pair of connected SMTP gateways.  That is to say, it provides
 channel security for the SMTP connections.  In a situation where the
 message goes directly from the client to the receiver's gateway, this
 may provide substantial security (though the receiver must still
 trust the gateway).  Protection is provided against replay attacks,
 since the data itself is protected and the packets cannot be
 Endpoint identification is a problem, however, unless the receiver's
 address can be directly cryptographically authenticated.  Sender
 identification is not generally available, since generally only the
 sender's machine is authenticated, not the sender himself.
 Furthermore, the identity of the sender simply appears in the From
 header of the message, so it is easily spoofable by the sender.
 Finally, unless the security policy is set extremely strictly, there
 is also an active downgrade to cleartext attack.

Rescorla & Korver Best Current Practice [Page 34] RFC 3552 Security Considerations Guidelines July 2003

 Another problem with IPsec as a security solution for SMTP is the
 lack of a standard IPsec API.  In order to take advantage of IPsec,
 applications in general need to be able to instruct the IPsec
 implementation about their security policies and discover what
 protection has been applied to their connections.  Without a standard
 API this is very difficult to do portably.
 Implementors of SMTP servers or SMTP administrators MUST NOT assume
 that IPsec will be available unless they have reason to believe that
 it will be (such as the existence of preexisting association between
 two machines).  However, it may be a reasonable procedure to attempt
 to create an IPsec association opportunistically to a peer server
 when mail is delivered.  Note that in cases where IPsec is used to
 provide a VPN tunnel between two sites, this is of substantial
 security value, particularly to the extent that confidentiality is
 provided, subject to the caveats mentioned above.  Also see
 [USEIPSEC] for general guidance on the applicability of IPsec. SMTP/TLS [NEW]

 SMTP can be combined with TLS as described in [STARTTLS].  This
 provides similar protection to that provided when using IPSEC.  Since
 TLS certificates typically contain the server's host name, recipient
 authentication may be slightly more obvious, but is still susceptible
 to DNS spoofing attacks.  Notably, common implementations of TLS
 contain a US exportable (and hence low security) mode.  Applications
 desiring high security should ensure that this mode is disabled.
 Protection is provided against replay attacks, since the data itself
 is protected and the packets cannot be replayed.  [Note:  The
 Security Considerations section of the SMTP over TLS document is
 quite good and bears reading as an example of how to do things.] S/MIME and PGP/MIME [NEW]

 S/MIME and PGP/MIME are both message oriented security protocols.
 They provide object security for individual messages.  With various
 settings, sender and recipient authentication and confidentiality may
 be provided.  More importantly, the identification is not of the
 sending and receiving machines, but rather of the sender and
 recipient themselves.  (Or, at least, of cryptographic keys
 corresponding to the sender and recipient.)  Consequently, end-to-end
 security may be obtained.  Note, however, that no protection is
 provided against replay attacks.  Note also that S/MIME and PGP/MIME
 generally provide identifying marks for both sender and receiver.
 Thus even when confidentiality is provided, traffic analysis is still

Rescorla & Korver Best Current Practice [Page 35] RFC 3552 Security Considerations Guidelines July 2003

6.1.3. Denial of Service [NEW]

 None of these security measures provides any real protection against
 denial of service.  SMTP connections can easily be used to tie up
 system resources in a number of ways, including excessive port
 consumption, excessive disk usage (email is typically delivered to
 disk files), and excessive memory consumption (sendmail, for
 instance, is fairly large, and typically forks a new process to deal
 with each message.)
 If transport- or application-layer security is used for SMTP
 connections, it is possible to mount a variety of attacks on
 individual connections using forged RSTs or other kinds of packet

6.2. VRRP

 The second example is from VRRP, the Virtual Router Redundance
 Protocol ([VRRP]).  We reproduce here the Security Considerations
 section from that document (with new section numbers).  Our comments
 are indented and prefaced with 'NOTE:'.

6.2.1. Security Considerations

 VRRP is designed for a range of internetworking environments that may
 employ different security policies.  The protocol includes several
 authentication methods ranging from no authentication, simple clear
 text passwords, and strong authentication using IP Authentication
 with MD5 HMAC.  The details on each approach including possible
 attacks and recommended environments follows.
 Independent of any authentication type VRRP includes a mechanism
 (setting TTL=255, checking on receipt) that protects against VRRP
 packets being injected from another remote network.  This limits most
 vulnerabilities to local attacks.
    NOTE: The security measures discussed in the following sections
    only provide various kinds of authentication.  No confidentiality
    is provided at all.  This should be explicitly described as
    outside the scope. No Authentication

 The use of this authentication type means that VRRP protocol
 exchanges are not authenticated.  This type of authentication SHOULD
 only be used in environments were there is minimal security risk and
 little chance for configuration errors (e.g., two VRRP routers on a

Rescorla & Korver Best Current Practice [Page 36] RFC 3552 Security Considerations Guidelines July 2003 Simple Text Password

 The use of this authentication type means that VRRP protocol
 exchanges are authenticated by a simple clear text password.
 This type of authentication is useful to protect against accidental
 misconfiguration of routers on a LAN.  It protects against routers
 inadvertently backing up another router.  A new router must first be
 configured with the correct password before it can run VRRP with
 another router.  This type of authentication does not protect against
 hostile attacks where the password can be learned by a node snooping
 VRRP packets on the LAN.  The Simple Text Authentication combined
 with the TTL check makes it difficult for a VRRP packet to be sent
 from another LAN to disrupt VRRP operation.
 This type of authentication is RECOMMENDED when there is minimal risk
 of nodes on a LAN actively disrupting VRRP operation.  If this type
 of authentication is used the user should be aware that this clear
 text password is sent frequently, and therefore should not be the
 same as any security significant password.
    NOTE: This section should be clearer.  The basic point is that no
    authentication and Simple Text are only useful for a very limited
    threat model, namely that none of the nodes on the local LAN are
    hostile.  The TTL check prevents hostile nodes off-LAN from posing
    as valid nodes, but nothing stops hostile nodes on-LAN from
    impersonating authorized nodes.  This is not a particularly
    realistic threat model in many situations.  In particular, it's
    extremely brittle: the compromise of any node the LAN allows
    reconfiguration of the VRRP nodes. IP Authentication Header

 The use of this authentication type means the VRRP protocol exchanges
 are authenticated using the mechanisms defined by the IP
 Authentication Header [AH] using [HMAC].  This provides strong
 protection against configuration errors, replay attacks, and packet
 This type of authentication is RECOMMENDED when there is limited
 control over the administration of nodes on a LAN.  While this type
 of authentication does protect the operation of VRRP, there are other
 types of attacks that may be employed on shared media links (e.g.,
 generation of bogus ARP replies) which are independent from VRRP and
 are not protected.

Rescorla & Korver Best Current Practice [Page 37] RFC 3552 Security Considerations Guidelines July 2003

    NOTE: It's a mistake to have AH be a RECOMMENDED in this context.
    Since AH is the only mechanism that protects VRRP against attack
    from other nodes on the same LAN, it should be a MUST for cases
    where there are untrusted nodes on the same network.  In any case,
    AH should be a MUST implement.
    NOTE: There's an important piece of security analysis that's only
    hinted at in this document, namely the cost/benefit tradeoff of
    VRRP authentication.
 [The rest of this section is NEW material]
 The threat that VRRP authentication is intended to prevent is an
 attacker arranging to be the VRRP master.  This would be done by
 joining the group (probably multiple times), gagging the master and
 then electing oneself master.  Such a node could then direct traffic
 in arbitrary undesirable ways.
 However, it is not necessary for an attacker to be the VRRP master to
 do this.  An attacker can do similar kinds of damage to the network
 by forging ARP packets or (on switched networks) fooling the switch
 VRRP authentication offers no real protection against these attacks.
 Unfortunately, authentication makes VRRP networks very brittle in the
 face of misconfiguration.  Consider what happens if two nodes are
 configured with different passwords.  Each will reject messages from
 the other and therefore both will attempt to be master.  This creates
 substantial network instability.
 This set of cost/benefit tradeoffs suggests that VRRP authentication
 is a bad idea, since the incremental security benefit is marginal but
 the incremental risk is high.  This judgment should be revisited if
 the current set of non-VRRP threats are removed.

7. Acknowledgments

 This document is heavily based on a note written by Ran Atkinson in
 1997.  That note was written after the IAB Security Workshop held in
 early 1997, based on input from everyone at that workshop.  Some of
 the specific text above was taken from Ran's original document, and
 some of that text was taken from an email message written by Fred
 Baker.  The other primary source for this document is specific
 comments received from Steve Bellovin.  Early review of this document
 was done by Lisa Dusseault and Mark Schertler.  Other useful comments
 were received from Bill Fenner, Ned Freed, Lawrence Greenfield, Steve
 Kent, Allison Mankin and Kurt Zeilenga.

Rescorla & Korver Best Current Practice [Page 38] RFC 3552 Security Considerations Guidelines July 2003

8. Normative References

 [AH]       Kent, S. and R. Atkinson, "IP Authentication Header", RFC
            2402, November 1998.
 [DNSSEC]   Eastlake, D., "Domain Name System Security Extensions",
            RFC 2535, March 1999.
 [ENCOPT]   Tso, T., "Telnet Data Encryption Option", RFC 2946,
            September, 2000.
 [ESP]      Kent, S. and R. Atkinson, "IP Encapsulating Security
            Payload (ESP)", RFC 2406, November 1998.
 [GSS]      Linn, J., "Generic Security Services Application Program
            Interface Version 2, Update 1", RFC 2743, January 2000.
 [HTTP]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
            Masinter, L., Leach, P. and T. Berners-Lee, "HyperText
            Transfer Protocol", RFC 2616, June 1999.
 [HTTPTLS]  Rescorla, E., "HTTP over TLS", RFC 2818, May 2000.
 [HMAC]     Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within
            ESP and AH", RFC 2403, November 1998.
 KERBEROS]  Kohl, J. and C. Neuman, "The Kerberos Network
            Authentication Service (V5)", RFC 1510, September 1993.
 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [OTP]      Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-Time
            Password System", STD 61, RFC 2289, February 1998.
 [PHOTURIS] Karn, P. and W. Simpson, "Photuris: Session-Key Management
            Protocol", RFC 2522, March 1999.
 [PKIX]     Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
            X.509 "Public Key Infrastructure Certificate and
            Certificate Restoration List (CRL) Profile", RFC 3280,
            April 2002.
 [RFC-2223] Postel J. and J. Reynolds, "Instructions to RFC Authors",
            RFC 2223, October 1997.
 [RFC-2505] Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs",
            BCP 30, RFC 2505, February 1999.

Rescorla & Korver Best Current Practice [Page 39] RFC 3552 Security Considerations Guidelines July 2003

 [RFC-2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
            April 2001.
 [SASL]     Myers, J., "Simple Authentication and Security Layer
            (SASL)", RFC 2222, October 1997.
 [SPKI]     Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas,
            B. and T. Ylonen, "SPKI Certificate Theory",  RFC 2693,
            September 1999.
 [SSH]      Ylonen, T., "SSH - Secure Login Connections Over the
            Internet", 6th USENIX Security Symposium, p. 37-42, July
 [SASLSMTP] Myers, J., "SMTP Service Extension for Authentication",
            RFC 2554, March 1999.
 [STARTTLS] Hoffman, P., "SMTP Service Extension for Secure SMTP over
            Transport Layer Security", RFC 3207, February 2002.
 [S-HTTP]   Rescorla, E. and A. Schiffman, "The Secure HyperText
            Transfer Protocol", RFC 2660, August 1999.
 [S/MIME]   Ramsdell, B., Editor, "S/MIME Version 3 Message
            Specification", RFC 2633, June 1999.
 [TELNET]   Postel, J. and J. Reynolds, "Telnet Protocol
            Specification", STD 8, RFC 854, May 1983.
 [TLS]      Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
            RFC 2246, January 1999.
 [TLSEXT]   Blake-Wilson, S., Nystrom, M., Hopwood, D. and J.
            Mikkelsen, "Transport Layer Security (TLS) Extensions",
            RFC 3546, May 2003.
 [TCPSYN]   "TCP SYN Flooding and IP Spoofing Attacks", CERT Advisory
            CA-1996-21, 19 September 1996, CERT.
 [UPGRADE]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
            HTTP/1.1", RFC 2817, May 2000.
 [URL]      Berners-Lee, T., Masinter, M. and M. McCahill, "Uniform
            Resource Locators (URL)", RFC 1738, December 1994.

Rescorla & Korver Best Current Practice [Page 40] RFC 3552 Security Considerations Guidelines July 2003

 [VRRP]     Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel,
            D., Hunt, P., Higginson, P., Shand, M. and A. Lindemn,
            "Virtual Router Redundancy Protocol", RFC 2338, April

9. Informative References

 [DDOS]     "Denial-Of-Service Tools" CERT Advisory CA-1999-17, 28
            December 1999, CERT
 [EKE]      Bellovin, S., Merritt, M., "Encrypted Key Exchange:
            Password-based protocols secure against dictionary
            attacks", Proceedings of the IEEE Symposium on Research in
            Security and Privacy, May 1992.
 [IDENT]    St. Johns, M. and M. Rose, "Identification Protocol", RFC
            1414, February 1993.
 [INTAUTH]  Haller, N. and R. Atkinson, "On Internet Authentication",
            RFC 1704, October 1994.
 [IPSPPROB] Bellovin, S. M., "Problem Areas for the IP Security
            Protocols", Proceedings of the Sixth Usenix UNIX Security
            Symposium, July 1996.
 [KLEIN]    Klein, D.V., "Foiling the Cracker: A Survey of and
            Improvements to Password Security",  1990.
 [NNTP]     Kantor, B. and P. Lapsley, "Network News Transfer
            Protocol", RFC 977, February 1986.
 [POP]      Myers, J. and M. Rose, "Post Office Protocol - Version 3",
            STD 53, RFC 1939, May 1996.
 [SEQNUM]   Morris, R.T., "A Weakness in the 4.2 BSD UNIX TCP/IP
            Software", AT&T Bell Laboratories, CSTR 117, 1985.
 [SOAP]     Box, D., Ehnebuske, D., Kakivaya, G., Layman, A.,
            Mendelsoh, N., Nielsen, H., Thatte, S., Winer, D., "Simple
            Object Access Protocol (SOAP) 1.1", May 2000.
 [SPEKE]    Jablon, D., "Strong Password-Only Authenticated Key
            Exchange", Computer Communication Review, ACM SIGCOMM,
            vol. 26, no. 5, pp. 5-26, October 1996.
 [SRP]      Wu T., "The Secure Remote Password Protocol", ISOC NDSS
            Symposium, 1998.

Rescorla & Korver Best Current Practice [Page 41] RFC 3552 Security Considerations Guidelines July 2003

 [USEIPSEC] Bellovin, S., "Guidelines for Mandating the Use of IPsec",
            Work in Progress.
 [WEP]      Borisov, N., Goldberg, I., Wagner, D., "Intercepting
            Mobile Communications: The Insecurity of 802.11",

10. Security Considerations

 This entire document is about security considerations.

Rescorla & Korver Best Current Practice [Page 42] RFC 3552 Security Considerations Guidelines July 2003

Appendix A.

 IAB Members at the time of this writing
 Harald Alvestrand
 Ran Atkinson
 Rob Austein
 Fred Baker
 Leslie Daigle
 Steve Deering
 Sally Floyd
 Ted Hardie
 Geoff Huston
 Charlie Kaufman
 James Kempf
 Eric Rescorla
 Mike St. Johns

Authors' Addresses

 Eric Rescorla
 RTFM, Inc.
 2439 Alvin Drive
 Mountain View, CA 94043
 Phone: (650)-320-8549
 Brian Korver
 Xythos Software, Inc.
 77 Maiden Lane, 6th Floor
 San Francisco, CA, 94108
 Phone: (415)-248-3800
 Internet Architecture Board

Rescorla & Korver Best Current Practice [Page 43] RFC 3552 Security Considerations Guidelines July 2003

Full Copyright Statement

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 or assist in its implementation may be prepared, copied, published
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 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Rescorla & Korver Best Current Practice [Page 44]

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