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rfc:rfc4513

Network Working Group R. Harrison, Ed. Request for Comments: 4513 Novell, Inc. Obsoletes: 2251, 2829, 2830 June 2006 Category: Standards Track

           Lightweight Directory Access Protocol (LDAP):
           Authentication Methods and Security Mechanisms

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This document describes authentication methods and security
 mechanisms of the Lightweight Directory Access Protocol (LDAP).  This
 document details establishment of Transport Layer Security (TLS)
 using the StartTLS operation.
 This document details the simple Bind authentication method including
 anonymous, unauthenticated, and name/password mechanisms and the
 Simple Authentication and Security Layer (SASL) Bind authentication
 method including the EXTERNAL mechanism.
 This document discusses various authentication and authorization
 states through which a session to an LDAP server may pass and the
 actions that trigger these state changes.
 This document, together with other documents in the LDAP Technical
 Specification (see Section 1 of the specification's road map),
 obsoletes RFC 2251, RFC 2829, and RFC 2830.

Harrison Standards Track [Page 1] RFC 4513 LDAP Authentication Methods June 2006

Table of Contents

 1. Introduction ....................................................4
    1.1. Relationship to Other Documents ............................6
    1.2. Conventions ................................................6
 2. Implementation Requirements .....................................7
 3. StartTLS Operation ..............................................8
    3.1.  TLS Establishment Procedures ..............................8
         3.1.1. StartTLS Request Sequencing .........................8
         3.1.2. Client Certificate ..................................9
         3.1.3. Server Identity Check ...............................9
                3.1.3.1. Comparison of DNS Names ...................10
                3.1.3.2. Comparison of IP Addresses ................11
                3.1.3.3. Comparison of Other subjectName Types .....11
         3.1.4. Discovery of Resultant Security Level ..............11
         3.1.5. Refresh of Server Capabilities Information .........11
    3.2.  Effect of TLS on Authorization State .....................12
    3.3. TLS Ciphersuites ..........................................12
 4. Authorization State ............................................13
 5. Bind Operation .................................................14
    5.1. Simple Authentication Method ..............................14
         5.1.1. Anonymous Authentication Mechanism of Simple Bind ..14
         5.1.2. Unauthenticated Authentication Mechanism of
                Simple Bind ........................................14
         5.1.3. Name/Password Authentication Mechanism of
                Simple Bind ........................................15
    5.2. SASL Authentication Method ................................16
         5.2.1. SASL Protocol Profile ..............................16
                5.2.1.1. SASL Service Name for LDAP ................16
                5.2.1.2. SASL Authentication Initiation and
                         Protocol Exchange .........................16
                5.2.1.3. Optional Fields ...........................17
                5.2.1.4. Octet Where Negotiated Security
                         Layers Take Effect ........................18
                5.2.1.5. Determination of Supported SASL
                         Mechanisms ................................18
                5.2.1.6. Rules for Using SASL Layers ...............19
                5.2.1.7. Support for Multiple Authentications ......19
                5.2.1.8. SASL Authorization Identities .............19
         5.2.2. SASL Semantics within LDAP .........................20
         5.2.3. SASL EXTERNAL Authentication Mechanism .............20
                5.2.3.1. Implicit Assertion ........................21
                5.2.3.2. Explicit Assertion ........................21
 6. Security Considerations ........................................21
    6.1. General LDAP Security Considerations ......................21
    6.2. StartTLS Security Considerations ..........................22
    6.3. Bind Operation Security Considerations ....................23
         6.3.1. Unauthenticated Mechanism Security Considerations ..23

Harrison Standards Track [Page 2] RFC 4513 LDAP Authentication Methods June 2006

         6.3.2. Name/Password Mechanism Security Considerations ....23
         6.3.3. Password-Related Security Considerations ...........23
         6.3.4. Hashed Password Security Considerations ............24
    6.4. SASL Security Considerations ..............................24
    6.5. Related Security Considerations ...........................25
 7. IANA Considerations ............................................25
 8. Acknowledgements ...............................................25
 9. Normative References ...........................................26
 10. Informative References ........................................27
 Appendix A. Authentication and Authorization Concepts .............28
    A.1. Access Control Policy .....................................28
    A.2. Access Control Factors ....................................28
    A.3. Authentication, Credentials, Identity .....................28
    A.4. Authorization Identity ....................................29
 Appendix B. Summary of Changes ....................................29
    B.1. Changes Made to RFC 2251 ..................................30
         B.1.1. Section 4.2.1 ("Sequencing of the Bind Request") ...30
         B.1.2. Section 4.2.2 ("Authentication and Other Security
                Services") .........................................30
    B.2. Changes Made to RFC 2829 ..................................30
         B.2.1. Section 4 ("Required security mechanisms") .........30
         B.2.2. Section 5.1 ("Anonymous authentication
                procedure") ........................................31
         B.2.3. Section 6 ("Password-based authentication") ........31
         B.2.4. Section 6.1 ("Digest authentication") ..............31
         B.2.5. Section 6.2 ("'simple' authentication choice under
                TLS encryption") ...................................31
         B.2.6. Section 6.3 ("Other authentication choices with
                TLS") ..............................................31
         B.2.7. Section 7.1 ("Certificate-based authentication
                with TLS") .........................................31
         B.2.8. Section 8 ("Other mechanisms") .....................32
         B.2.9. Section 9 ("Authorization Identity") ...............32
         B.2.10. Section 10 ("TLS Ciphersuites") ...................32
    B.3. Changes Made to RFC 2830 ..................................32
         B.3.1. Section 3.6 ("Server Identity Check") ..............32
         B.3.2. Section 3.7 ("Refresh of Server Capabilities
                Information") ......................................33
         B.3.3. Section 5 ("Effects of TLS on a Client's
                Authorization Identity") ...........................33
         B.3.4. Section 5.2 ("TLS Connection Closure Effects") .....33

Harrison Standards Track [Page 3] RFC 4513 LDAP Authentication Methods June 2006

1. Introduction

 The Lightweight Directory Access Protocol (LDAP) [RFC4510] is a
 powerful protocol for accessing directories.  It offers means of
 searching, retrieving, and manipulating directory content and ways to
 access a rich set of security functions.
 It is vital that these security functions be interoperable among all
 LDAP clients and servers on the Internet; therefore there has to be a
 minimum subset of security functions that is common to all
 implementations that claim LDAP conformance.
 Basic threats to an LDAP directory service include (but are not
 limited to):
 (1) Unauthorized access to directory data via data-retrieval
     operations.
 (2) Unauthorized access to directory data by monitoring access of
     others.
 (3) Unauthorized access to reusable client authentication information
     by monitoring access of others.
 (4) Unauthorized modification of directory data.
 (5) Unauthorized modification of configuration information.
 (6) Denial of Service: Use of resources (commonly in excess) in a
     manner intended to deny service to others.
 (7) Spoofing: Tricking a user or client into believing that
     information came from the directory when in fact it did not,
     either by modifying data in transit or misdirecting the client's
     transport connection.  Tricking a user or client into sending
     privileged information to a hostile entity that appears to be the
     directory server but is not.  Tricking a directory server into
     believing that information came from a particular client when in
     fact it came from a hostile entity.
 (8) Hijacking: An attacker seizes control of an established protocol
     session.
 Threats (1), (4), (5), (6), (7), and (8) are active attacks.  Threats
 (2) and (3) are passive attacks.

Harrison Standards Track [Page 4] RFC 4513 LDAP Authentication Methods June 2006

 Threats (1), (4), (5), and (6) are due to hostile clients.  Threats
 (2), (3), (7), and (8) are due to hostile agents on the path between
 client and server or hostile agents posing as a server, e.g., IP
 spoofing.
 LDAP offers the following security mechanisms:
 (1) Authentication by means of the Bind operation.  The Bind
     operation provides a simple method that supports anonymous,
     unauthenticated, and name/password mechanisms, and the Simple
     Authentication and Security Layer (SASL) method, which supports a
     wide variety of authentication mechanisms.
 (2) Mechanisms to support vendor-specific access control facilities
     (LDAP does not offer a standard access control facility).
 (3) Data integrity service by means of security layers in Transport
     Layer Security (TLS) or SASL mechanisms.
 (4) Data confidentiality service by means of security layers in TLS
     or SASL mechanisms.
 (5) Server resource usage limitation by means of administrative
     limits configured on the server.
 (6) Server authentication by means of the TLS protocol or SASL
     mechanisms.
 LDAP may also be protected by means outside the LDAP protocol, e.g.,
 with IP layer security [RFC4301].
 Experience has shown that simply allowing implementations to pick and
 choose the security mechanisms that will be implemented is not a
 strategy that leads to interoperability.  In the absence of mandates,
 clients will continue to be written that do not support any security
 function supported by the server, or worse, they will only support
 mechanisms that provide inadequate security for most circumstances.
 It is desirable to allow clients to authenticate using a variety of
 mechanisms including mechanisms where identities are represented as
 distinguished names [X.501][RFC4512], in string form [RFC4514], or as
 used in different systems (e.g., simple user names [RFC4013]).
 Because some authentication mechanisms transmit credentials in plain
 text form, and/or do not provide data security services and/or are
 subject to passive attacks, it is necessary to ensure secure
 interoperability by identifying a mandatory-to-implement mechanism
 for establishing transport-layer security services.

Harrison Standards Track [Page 5] RFC 4513 LDAP Authentication Methods June 2006

 The set of security mechanisms provided in LDAP and described in this
 document is intended to meet the security needs for a wide range of
 deployment scenarios and still provide a high degree of
 interoperability among various LDAP implementations and deployments.

1.1. Relationship to Other Documents

 This document is an integral part of the LDAP Technical Specification
 [RFC4510].
 This document, together with [RFC4510], [RFC4511], and [RFC4512],
 obsoletes RFC 2251 in its entirety.  Sections 4.2.1 (portions) and
 4.2.2 of RFC 2251 are obsoleted by this document.  Appendix B.1
 summarizes the substantive changes made to RFC 2251 by this document.
 This document obsoletes RFC 2829 in its entirety.  Appendix B.2
 summarizes the substantive changes made to RFC 2829 by this document.
 Sections 2 and 4 of RFC 2830 are obsoleted by [RFC4511].  The
 remainder of RFC 2830 is obsoleted by this document.  Appendix B.3
 summarizes the substantive changes made to RFC 2830 by this document.

1.2. Conventions

 The key words "MUST", "MUST NOT", "SHALL", "SHOULD", "SHOULD NOT",
 "MAY", and "OPTIONAL" in this document are to be interpreted as
 described in RFC 2119 [RFC2119].
 The term "user" represents any human or application entity that is
 accessing the directory using a directory client.  A directory client
 (or client) is also known as a directory user agent (DUA).
 The term "transport connection" refers to the underlying transport
 services used to carry the protocol exchange, as well as associations
 established by these services.
 The term "TLS layer" refers to TLS services used in providing
 security services, as well as associations established by these
 services.
 The term "SASL layer" refers to SASL services used in providing
 security services, as well as associations established by these
 services.
 The term "LDAP message layer" refers to the LDAP Message (PDU)
 services used in providing directory services, as well as
 associations established by these services.

Harrison Standards Track [Page 6] RFC 4513 LDAP Authentication Methods June 2006

 The term "LDAP session" refers to combined services (transport
 connection, TLS layer, SASL layer, LDAP message layer) and their
 associations.
 In general, security terms in this document are used consistently
 with the definitions provided in [RFC2828].  In addition, several
 terms and concepts relating to security, authentication, and
 authorization are presented in Appendix A of this document.  While
 the formal definition of these terms and concepts is outside the
 scope of this document, an understanding of them is prerequisite to
 understanding much of the material in this document.  Readers who are
 unfamiliar with security-related concepts are encouraged to review
 Appendix A before reading the remainder of this document.

2. Implementation Requirements

 LDAP server implementations MUST support the anonymous authentication
 mechanism of the simple Bind method (Section 5.1.1).
 LDAP implementations that support any authentication mechanism other
 than the anonymous authentication mechanism of the simple Bind method
 MUST support the name/password authentication mechanism of the simple
 Bind method (Section 5.1.3) and MUST be capable of protecting this
 name/password authentication using TLS as established by the StartTLS
 operation (Section 3).
 Implementations SHOULD disallow the use of the name/password
 authentication mechanism by default when suitable data security
 services are not in place, and they MAY provide other suitable data
 security services for use with this authentication mechanism.
 Implementations MAY support additional authentication mechanisms.
 Some of these mechanisms are discussed below.
 LDAP server implementations SHOULD support client assertion of
 authorization identity via the SASL EXTERNAL mechanism (Section
 5.2.3).
 LDAP server implementations that support no authentication mechanism
 other than the anonymous mechanism of the simple bind method SHOULD
 support use of TLS as established by the StartTLS operation (Section
 3).  (Other servers MUST support TLS per the second paragraph of this
 section.)

Harrison Standards Track [Page 7] RFC 4513 LDAP Authentication Methods June 2006

 Implementations supporting TLS MUST support the
 TLS_RSA_WITH_3DES_EDE_CBC_SHA ciphersuite and SHOULD support the
 TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA ciphersuite.  Support for the
 latter ciphersuite is recommended to encourage interoperability with
 implementations conforming to earlier LDAP StartTLS specifications.

3. StartTLS Operation

 The Start Transport Layer Security (StartTLS) operation defined in
 Section 4.14 of [RFC4511] provides the ability to establish TLS
 [RFC4346] in an LDAP session.
 The goals of using the TLS protocol with LDAP are to ensure data
 confidentiality and integrity, and to optionally provide for
 authentication.  TLS expressly provides these capabilities, although
 the authentication services of TLS are available to LDAP only in
 combination with the SASL EXTERNAL authentication method (see Section
 5.2.3), and then only if the SASL EXTERNAL implementation chooses to
 make use of the TLS credentials.

3.1. TLS Establishment Procedures

 This section describes the overall procedures clients and servers
 must follow for TLS establishment.  These procedures take into
 consideration various aspects of the TLS layer including discovery of
 resultant security level and assertion of the client's authorization
 identity.

3.1.1. StartTLS Request Sequencing

 A client may send the StartTLS extended request at any time after
 establishing an LDAP session, except:
  1. when TLS is currently established on the session,
  2. when a multi-stage SASL negotiation is in progress on the

session, or

  1. when there are outstanding responses for operation requests

previously issued on the session.

 As described in [RFC4511], Section 4.14.1, a (detected) violation of
 any of these requirements results in a return of the operationsError
 resultCode.
 Client implementers should ensure that they strictly follow these
 operation sequencing requirements to prevent interoperability issues.
 Operational experience has shown that violating these requirements

Harrison Standards Track [Page 8] RFC 4513 LDAP Authentication Methods June 2006

 causes interoperability issues because there are race conditions that
 prevent servers from detecting some violations of these requirements
 due to factors such as server hardware speed and network latencies.
 There is no general requirement that the client have or have not
 already performed a Bind operation (Section 5) before sending a
 StartTLS operation request; however, where a client intends to
 perform both a Bind operation and a StartTLS operation, it SHOULD
 first perform the StartTLS operation so that the Bind request and
 response messages are protected by the data security services
 established by the StartTLS operation.

3.1.2. Client Certificate

 If an LDAP server requests or demands that a client provide a user
 certificate during TLS negotiation and the client does not present a
 suitable user certificate (e.g., one that can be validated), the
 server may use a local security policy to determine whether to
 successfully complete TLS negotiation.
 If a client that has provided a suitable certificate subsequently
 performs a Bind operation using the SASL EXTERNAL authentication
 mechanism (Section 5.2.3), information in the certificate may be used
 by the server to identify and authenticate the client.

3.1.3. Server Identity Check

 In order to prevent man-in-the-middle attacks, the client MUST verify
 the server's identity (as presented in the server's Certificate
 message).  In this section, the client's understanding of the
 server's identity (typically the identity used to establish the
 transport connection) is called the "reference identity".
 The client determines the type (e.g., DNS name or IP address) of the
 reference identity and performs a comparison between the reference
 identity and each subjectAltName value of the corresponding type
 until a match is produced.  Once a match is produced, the server's
 identity has been verified, and the server identity check is
 complete.  Different subjectAltName types are matched in different
 ways.  Sections 3.1.3.1 - 3.1.3.3 explain how to compare values of
 various subjectAltName types.
 The client may map the reference identity to a different type prior
 to performing a comparison.  Mappings may be performed for all
 available subjectAltName types to which the reference identity can be
 mapped; however, the reference identity should only be mapped to
 types for which the mapping is either inherently secure (e.g.,
 extracting the DNS name from a URI to compare with a subjectAltName

Harrison Standards Track [Page 9] RFC 4513 LDAP Authentication Methods June 2006

 of type dNSName) or for which the mapping is performed in a secure
 manner (e.g., using DNSSEC, or using user- or admin-configured host-
 to-address/address-to-host lookup tables).
 The server's identity may also be verified by comparing the reference
 identity to the Common Name (CN) [RFC4519] value in the leaf Relative
 Distinguished Name (RDN) of the subjectName field of the server's
 certificate.  This comparison is performed using the rules for
 comparison of DNS names in Section 3.1.3.1, below, with the exception
 that no wildcard matching is allowed.  Although the use of the Common
 Name value is existing practice, it is deprecated, and Certification
 Authorities are encouraged to provide subjectAltName values instead.
 Note that the TLS implementation may represent DNs in certificates
 according to X.500 or other conventions.  For example, some X.500
 implementations order the RDNs in a DN using a left-to-right (most
 significant to least significant) convention instead of LDAP's
 right-to-left convention.
 If the server identity check fails, user-oriented clients SHOULD
 either notify the user (clients may give the user the opportunity to
 continue with the LDAP session in this case) or close the transport
 connection and indicate that the server's identity is suspect.
 Automated clients SHOULD close the transport connection and then
 return or log an error indicating that the server's identity is
 suspect or both.
 Beyond the server identity check described in this section, clients
 should be prepared to do further checking to ensure that the server
 is authorized to provide the service it is requested to provide.  The
 client may need to make use of local policy information in making
 this determination.

3.1.3.1. Comparison of DNS Names

 If the reference identity is an internationalized domain name,
 conforming implementations MUST convert it to the ASCII Compatible
 Encoding (ACE) format as specified in Section 4 of RFC 3490 [RFC3490]
 before comparison with subjectAltName values of type dNSName.
 Specifically, conforming implementations MUST perform the conversion
 operation specified in Section 4 of RFC 3490 as follows:
  • in step 1, the domain name SHALL be considered a "stored

string";

  • in step 3, set the flag called "UseSTD3ASCIIRules";
  • in step 4, process each label with the "ToASCII" operation; and
  • in step 5, change all label separators to U+002E (full stop).

Harrison Standards Track [Page 10] RFC 4513 LDAP Authentication Methods June 2006

 After performing the "to-ASCII" conversion, the DNS labels and names
 MUST be compared for equality according to the rules specified in
 Section 3 of RFC3490.
 The '*' (ASCII 42) wildcard character is allowed in subjectAltName
 values of type dNSName, and then only as the left-most (least
 significant) DNS label in that value.  This wildcard matches any
 left-most DNS label in the server name.  That is, the subject
 *.example.com matches the server names a.example.com and
 b.example.com, but does not match example.com or a.b.example.com.

3.1.3.2. Comparison of IP Addresses

 When the reference identity is an IP address, the identity MUST be
 converted to the "network byte order" octet string representation
 [RFC791][RFC2460].  For IP Version 4, as specified in RFC 791, the
 octet string will contain exactly four octets.  For IP Version 6, as
 specified in RFC 2460, the octet string will contain exactly sixteen
 octets.  This octet string is then compared against subjectAltName
 values of type iPAddress.  A match occurs if the reference identity
 octet string and value octet strings are identical.

3.1.3.3. Comparison of Other subjectName Types

 Client implementations MAY support matching against subjectAltName
 values of other types as described in other documents.

3.1.4. Discovery of Resultant Security Level

 After a TLS layer is established in an LDAP session, both parties are
 to each independently decide whether or not to continue based on
 local policy and the security level achieved.  If either party
 decides that the security level is inadequate for it to continue, it
 SHOULD remove the TLS layer immediately after the TLS (re)negotiation
 has completed (see [RFC4511], Section 4.14.3, and Section 3.2 below).
 Implementations may reevaluate the security level at any time and,
 upon finding it inadequate, should remove the TLS layer.

3.1.5. Refresh of Server Capabilities Information

 After a TLS layer is established in an LDAP session, the client
 SHOULD discard or refresh all information about the server that it
 obtained prior to the initiation of the TLS negotiation and that it
 did not obtain through secure mechanisms.  This protects against
 man-in-the-middle attacks that may have altered any server
 capabilities information retrieved prior to TLS layer installation.

Harrison Standards Track [Page 11] RFC 4513 LDAP Authentication Methods June 2006

 The server may advertise different capabilities after installing a
 TLS layer.  In particular, the value of 'supportedSASLMechanisms' may
 be different after a TLS layer has been installed (specifically, the
 EXTERNAL and PLAIN [PLAIN] mechanisms are likely to be listed only
 after a TLS layer has been installed).

3.2. Effect of TLS on Authorization State

 The establishment, change, and/or closure of TLS may cause the
 authorization state to move to a new state.  This is discussed
 further in Section 4.

3.3. TLS Ciphersuites

 Several issues should be considered when selecting TLS ciphersuites
 that are appropriate for use in a given circumstance.  These issues
 include the following:
  1. The ciphersuite's ability to provide adequate confidentiality

protection for passwords and other data sent over the transport

      connection.  Client and server implementers should recognize
      that some TLS ciphersuites provide no confidentiality
      protection, while other ciphersuites that do provide
      confidentiality protection may be vulnerable to being cracked
      using brute force methods, especially in light of ever-
      increasing CPU speeds that reduce the time needed to
      successfully mount such attacks.
  1. Client and server implementers should carefully consider the

value of the password or data being protected versus the level

      of confidentiality protection provided by the ciphersuite to
      ensure that the level of protection afforded by the ciphersuite
      is appropriate.
  1. The ciphersuite's vulnerability (or lack thereof) to man-in-the-

middle attacks. Ciphersuites vulnerable to man-in-the-middle

      attacks SHOULD NOT be used to protect passwords or sensitive
      data, unless the network configuration is such that the danger
      of a man-in-the-middle attack is negligible.
  1. After a TLS negotiation (either initial or subsequent) is

completed, both protocol peers should independently verify that

      the security services provided by the negotiated ciphersuite are
      adequate for the intended use of the LDAP session.  If they are
      not, the TLS layer should be closed.

Harrison Standards Track [Page 12] RFC 4513 LDAP Authentication Methods June 2006

4. Authorization State

 Every LDAP session has an associated authorization state.  This state
 is comprised of numerous factors such as what (if any) authentication
 state has been established, how it was established, and what security
 services are in place.  Some factors may be determined and/or
 affected by protocol events (e.g., Bind, StartTLS, or TLS closure),
 and some factors may be determined by external events (e.g., time of
 day or server load).
 While it is often convenient to view authorization state in
 simplistic terms (as we often do in this technical specification)
 such as "an anonymous state", it is noted that authorization systems
 in LDAP implementations commonly involve many factors that
 interrelate in complex manners.
 Authorization in LDAP is a local matter.  One of the key factors in
 making authorization decisions is authorization identity.  The Bind
 operation (defined in Section 4.2 of [RFC4511] and discussed further
 in Section 5 below) allows information to be exchanged between the
 client and server to establish an authorization identity for the LDAP
 session.  The Bind operation may also be used to move the LDAP
 session to an anonymous authorization state (see Section 5.1.1).
 Upon initial establishment of the LDAP session, the session has an
 anonymous authorization identity.  Among other things this implies
 that the client need not send a BindRequest in the first PDU of the
 LDAP message layer.  The client may send any operation request prior
 to performing a Bind operation, and the server MUST treat it as if it
 had been performed after an anonymous Bind operation (Section 5.1.1).
 Upon receipt of a Bind request, the server immediately moves the
 session to an anonymous authorization state.  If the Bind request is
 successful, the session is moved to the requested authentication
 state with its associated authorization state.  Otherwise, the
 session remains in an anonymous state.
 It is noted that other events both internal and external to LDAP may
 result in the authentication and authorization states being moved to
 an anonymous one.  For instance, the establishment, change, or
 closure of data security services may result in a move to an
 anonymous state, or the user's credential information (e.g.,
 certificate) may have expired.  The former is an example of an event
 internal to LDAP, whereas the latter is an example of an event
 external to LDAP.

Harrison Standards Track [Page 13] RFC 4513 LDAP Authentication Methods June 2006

5. Bind Operation

 The Bind operation ([RFC4511], Section 4.2) allows authentication
 information to be exchanged between the client and server to
 establish a new authorization state.
 The Bind request typically specifies the desired authentication
 identity.  Some Bind mechanisms also allow the client to specify the
 authorization identity.  If the authorization identity is not
 specified, the server derives it from the authentication identity in
 an implementation-specific manner.
 If the authorization identity is specified, the server MUST verify
 that the client's authentication identity is permitted to assume
 (e.g., proxy for) the asserted authorization identity.  The server
 MUST reject the Bind operation with an invalidCredentials resultCode
 in the Bind response if the client is not so authorized.

5.1. Simple Authentication Method

 The simple authentication method of the Bind Operation provides three
 authentication mechanisms:
  1. An anonymous authentication mechanism (Section 5.1.1).
  1. An unauthenticated authentication mechanism (Section 5.1.2).
  1. A name/password authentication mechanism using credentials

consisting of a name (in the form of an LDAP distinguished name

      [RFC4514]) and a password (Section 5.1.3).

5.1.1. Anonymous Authentication Mechanism of Simple Bind

 An LDAP client may use the anonymous authentication mechanism of the
 simple Bind method to explicitly establish an anonymous authorization
 state by sending a Bind request with a name value of zero length and
 specifying the simple authentication choice containing a password
 value of zero length.

5.1.2. Unauthenticated Authentication Mechanism of Simple Bind

 An LDAP client may use the unauthenticated authentication mechanism
 of the simple Bind method to establish an anonymous authorization
 state by sending a Bind request with a name value (a distinguished
 name in LDAP string form [RFC4514] of non-zero length) and specifying
 the simple authentication choice containing a password value of zero
 length.

Harrison Standards Track [Page 14] RFC 4513 LDAP Authentication Methods June 2006

 The distinguished name value provided by the client is intended to be
 used for trace (e.g., logging) purposes only.  The value is not to be
 authenticated or otherwise validated (including verification that the
 DN refers to an existing directory object).  The value is not to be
 used (directly or indirectly) for authorization purposes.
 Unauthenticated Bind operations can have significant security issues
 (see Section 6.3.1).  In particular, users intending to perform
 Name/Password Authentication may inadvertently provide an empty
 password and thus cause poorly implemented clients to request
 Unauthenticated access.  Clients SHOULD be implemented to require
 user selection of the Unauthenticated Authentication Mechanism by
 means other than user input of an empty password.  Clients SHOULD
 disallow an empty password input to a Name/Password Authentication
 user interface.  Additionally, Servers SHOULD by default fail
 Unauthenticated Bind requests with a resultCode of
 unwillingToPerform.

5.1.3. Name/Password Authentication Mechanism of Simple Bind

 An LDAP client may use the name/password authentication mechanism of
 the simple Bind method to establish an authenticated authorization
 state by sending a Bind request with a name value (a distinguished
 name in LDAP string form [RFC4514] of non-zero length) and specifying
 the simple authentication choice containing an OCTET STRING password
 value of non-zero length.
 Servers that map the DN sent in the Bind request to a directory entry
 with an associated set of one or more passwords used with this
 mechanism will compare the presented password to that set of
 passwords.  The presented password is considered valid if it matches
 any member of this set.
 A resultCode of invalidDNSyntax indicates that the DN sent in the
 name value is syntactically invalid.  A resultCode of
 invalidCredentials indicates that the DN is syntactically correct but
 not valid for purposes of authentication, that the password is not
 valid for the DN, or that the server otherwise considers the
 credentials invalid.  A resultCode of success indicates that the
 credentials are valid and that the server is willing to provide
 service to the entity these credentials identify.
 Server behavior is undefined for Bind requests specifying the
 name/password authentication mechanism with a zero-length name value
 and a password value of non-zero length.

Harrison Standards Track [Page 15] RFC 4513 LDAP Authentication Methods June 2006

 The name/password authentication mechanism of the simple Bind method
 is not suitable for authentication in environments without
 confidentiality protection.

5.2. SASL Authentication Method

 The sasl authentication method of the Bind Operation provides
 facilities for using any SASL mechanism including authentication
 mechanisms and other services (e.g., data security services).

5.2.1. SASL Protocol Profile

 LDAP allows authentication via any SASL mechanism [RFC4422].  As LDAP
 includes native anonymous and name/password (plain text)
 authentication methods, the ANONYMOUS [RFC4505] and PLAIN [PLAIN]
 SASL mechanisms are typically not used with LDAP.
 Each protocol that utilizes SASL services is required to supply
 certain information profiling the way they are exposed through the
 protocol ([RFC4422], Section 4).  This section explains how each of
 these profiling requirements is met by LDAP.

5.2.1.1. SASL Service Name for LDAP

 The SASL service name for LDAP is "ldap", which has been registered
 with the IANA as a SASL service name.

5.2.1.2. SASL Authentication Initiation and Protocol Exchange

 SASL authentication is initiated via a BindRequest message
 ([RFC4511], Section 4.2) with the following parameters:
  1. The version is 3.
  2. The AuthenticationChoice is sasl.
  3. The mechanism element of the SaslCredentials sequence contains

the value of the desired SASL mechanism.

  1. The optional credentials field of the SaslCredentials sequence

MAY be used to provide an initial client response for mechanisms

      that are defined to have the client send data first (see
      [RFC4422], Sections 3 and 5).
 In general, a SASL authentication protocol exchange consists of a
 series of server challenges and client responses, the contents of
 which are specific to and defined by the SASL mechanism.  Thus, for
 some SASL authentication mechanisms, it may be necessary for the
 client to respond to one or more server challenges by sending
 BindRequest messages multiple times.  A challenge is indicated by the
 server sending a BindResponse message with the resultCode set to

Harrison Standards Track [Page 16] RFC 4513 LDAP Authentication Methods June 2006

 saslBindInProgress.  This indicates that the server requires the
 client to send a new BindRequest message with the same SASL mechanism
 to continue the authentication process.
 To the LDAP message layer, these challenges and responses are opaque
 binary tokens of arbitrary length.  LDAP servers use the
 serverSaslCreds field (an OCTET STRING) in a BindResponse message to
 transmit each challenge.  LDAP clients use the credentials field (an
 OCTET STRING) in the SaslCredentials sequence of a BindRequest
 message to transmit each response.  Note that unlike some Internet
 protocols where SASL is used, LDAP is not text based and does not
 Base64-transform these challenge and response values.
 Clients sending a BindRequest message with the sasl choice selected
 SHOULD send a zero-length value in the name field.  Servers receiving
 a BindRequest message with the sasl choice selected SHALL ignore any
 value in the name field.
 A client may abort a SASL Bind negotiation by sending a BindRequest
 message with a different value in the mechanism field of
 SaslCredentials or with an AuthenticationChoice other than sasl.
 If the client sends a BindRequest with the sasl mechanism field as an
 empty string, the server MUST return a BindResponse with a resultCode
 of authMethodNotSupported.  This will allow the client to abort a
 negotiation if it wishes to try again with the same SASL mechanism.
 The server indicates completion of the SASL challenge-response
 exchange by responding with a BindResponse in which the resultCode
 value is not saslBindInProgress.
 The serverSaslCreds field in the BindResponse can be used to include
 an optional challenge with a success notification for mechanisms that
 are defined to have the server send additional data along with the
 indication of successful completion.

5.2.1.3. Optional Fields

 As discussed above, LDAP provides an optional field for carrying an
 initial response in the message initiating the SASL exchange and
 provides an optional field for carrying additional data in the
 message indicating the outcome of the authentication exchange.  As
 the mechanism-specific content in these fields may be zero length,
 SASL requires protocol specifications to detail how an empty field is
 distinguished from an absent field.

Harrison Standards Track [Page 17] RFC 4513 LDAP Authentication Methods June 2006

 Zero-length initial response data is distinguished from no initial
 response data in the initiating message, a BindRequest PDU, by the
 presence of the SaslCredentials.credentials OCTET STRING (of length
 zero) in that PDU.  If the client does not intend to send an initial
 response with the BindRequest initiating the SASL exchange, it MUST
 omit the SaslCredentials.credentials OCTET STRING (rather than
 include an zero-length OCTET STRING).
 Zero-length additional data is distinguished from no additional
 response data in the outcome message, a BindResponse PDU, by the
 presence of the serverSaslCreds OCTET STRING (of length zero) in that
 PDU.  If a server does not intend to send additional data in the
 BindResponse message indicating outcome of the exchange, the server
 SHALL omit the serverSaslCreds OCTET STRING (rather than including a
 zero-length OCTET STRING).

5.2.1.4. Octet Where Negotiated Security Layers Take Effect

 SASL layers take effect following the transmission by the server and
 reception by the client of the final BindResponse in the SASL
 exchange with a resultCode of success.
 Once a SASL layer providing data integrity or confidentiality
 services takes effect, the layer remains in effect until a new layer
 is installed (i.e., at the first octet following the final
 BindResponse of the Bind operation that caused the new layer to take
 effect).  Thus, an established SASL layer is not affected by a failed
 or non-SASL Bind.

5.2.1.5. Determination of Supported SASL Mechanisms

 Clients may determine the SASL mechanisms a server supports by
 reading the 'supportedSASLMechanisms' attribute from the root DSE
 (DSA-Specific Entry) ([RFC4512], Section 5.1).  The values of this
 attribute, if any, list the mechanisms the server supports in the
 current LDAP session state.  LDAP servers SHOULD allow all clients --
 even those with an anonymous authorization -- to retrieve the
 'supportedSASLMechanisms' attribute of the root DSE both before and
 after the SASL authentication exchange.  The purpose of the latter is
 to allow the client to detect possible downgrade attacks (see Section
 6.4 and [RFC4422], Section 6.1.2).
 Because SASL mechanisms provide critical security functions, clients
 and servers should be configurable to specify what mechanisms are
 acceptable and allow only those mechanisms to be used.  Both clients
 and servers must confirm that the negotiated security level meets
 their requirements before proceeding to use the session.

Harrison Standards Track [Page 18] RFC 4513 LDAP Authentication Methods June 2006

5.2.1.6. Rules for Using SASL Layers

 Upon installing a SASL layer, the client SHOULD discard or refresh
 all information about the server that it obtained prior to the
 initiation of the SASL negotiation and that it did not obtain through
 secure mechanisms.
 If a lower-level security layer (such as TLS) is installed, any SASL
 layer SHALL be layered on top of such security layers regardless of
 the order of their negotiation.  In all other respects, the SASL
 layer and other security layers act independently, e.g., if both a
 TLS layer and a SASL layer are in effect, then removing the TLS layer
 does not affect the continuing service of the SASL layer.

5.2.1.7. Support for Multiple Authentications

 LDAP supports multiple SASL authentications as defined in [RFC4422],
 Section 4.

5.2.1.8. SASL Authorization Identities

 Some SASL mechanisms allow clients to request a desired authorization
 identity for the LDAP session ([RFC4422], Section 3.4).  The decision
 to allow or disallow the current authentication identity to have
 access to the requested authorization identity is a matter of local
 policy.  The authorization identity is a string of UTF-8 [RFC3629]
 encoded [Unicode] characters corresponding to the following Augmented
 Backus-Naur Form (ABNF) [RFC4234] grammar:
    authzId = dnAuthzId / uAuthzId
    ; distinguished-name-based authz id
    dnAuthzId =  "dn:" distinguishedName
    ; unspecified authorization id, UTF-8 encoded
    uAuthzId = "u:" userid
    userid = *UTF8 ; syntax unspecified
 where the distinguishedName rule is defined in Section 3 of [RFC4514]
 and the UTF8 rule is defined in Section 1.4 of [RFC4512].
 The dnAuthzId choice is used to assert authorization identities in
 the form of a distinguished name to be matched in accordance with the
 distinguishedNameMatch matching rule ([RFC4517], Section 4.2.15).
 There is no requirement that the asserted distinguishedName value be
 that of an entry in the directory.

Harrison Standards Track [Page 19] RFC 4513 LDAP Authentication Methods June 2006

 The uAuthzId choice allows clients to assert an authorization
 identity that is not in distinguished name form.  The format of
 userid is defined only as a sequence of UTF-8 [RFC3629] encoded
 [Unicode] characters, and any further interpretation is a local
 matter.  For example, the userid could identify a user of a specific
 directory service, be a login name, or be an email address.  A
 uAuthzId SHOULD NOT be assumed to be globally unique.  To compare
 uAuthzId values, each uAuthzId value MUST be prepared as a "query"
 string ([RFC3454], Section 7) using the SASLprep [RFC4013] algorithm,
 and then the two values are compared octet-wise.
 The above grammar is extensible.  The authzId production may be
 extended to support additional forms of identities.  Each form is
 distinguished by its unique prefix (see Section 3.12 of [RFC4520] for
 registration requirements).

5.2.2. SASL Semantics within LDAP

 Implementers must take care to maintain the semantics of SASL
 specifications when handling data that has different semantics in the
 LDAP protocol.
 For example, the SASL DIGEST-MD5 authentication mechanism
 [DIGEST-MD5] utilizes an authentication identity and a realm that are
 syntactically simple strings and semantically simple username
 [RFC4013] and realm values.  These values are not LDAP DNs, and there
 is no requirement that they be represented or treated as such.

5.2.3. SASL EXTERNAL Authentication Mechanism

 A client can use the SASL EXTERNAL ([RFC4422], Appendix A) mechanism
 to request the LDAP server to authenticate and establish a resulting
 authorization identity using security credentials exchanged by a
 lower security layer (such as by TLS authentication).  If the
 client's authentication credentials have not been established at a
 lower security layer, the SASL EXTERNAL Bind MUST fail with a
 resultCode of inappropriateAuthentication.  Although this situation
 has the effect of leaving the LDAP session in an anonymous state
 (Section 4), the state of any installed security layer is unaffected.
 A client may either request that its authorization identity be
 automatically derived from its authentication credentials exchanged
 at a lower security layer, or it may explicitly provide a desired
 authorization identity.  The former is known as an implicit
 assertion, and the latter as an explicit assertion.

Harrison Standards Track [Page 20] RFC 4513 LDAP Authentication Methods June 2006

5.2.3.1. Implicit Assertion

 An implicit authorization identity assertion is performed by invoking
 a Bind request of the SASL form using the EXTERNAL mechanism name
 that does not include the optional credentials field (found within
 the SaslCredentials sequence in the BindRequest).  The server will
 derive the client's authorization identity from the authentication
 identity supplied by a security layer (e.g., a public key certificate
 used during TLS layer installation) according to local policy.  The
 underlying mechanics of how this is accomplished are implementation
 specific.

5.2.3.2. Explicit Assertion

 An explicit authorization identity assertion is performed by invoking
 a Bind request of the SASL form using the EXTERNAL mechanism name
 that includes the credentials field (found within the SaslCredentials
 sequence in the BindRequest).  The value of the credentials field (an
 OCTET STRING) is the asserted authorization identity and MUST be
 constructed as documented in Section 5.2.1.8.

6. Security Considerations

 Security issues are discussed throughout this document.  The
 unsurprising conclusion is that security is an integral and necessary
 part of LDAP.  This section discusses a number of LDAP-related
 security considerations.

6.1. General LDAP Security Considerations

 LDAP itself provides no security or protection from accessing or
 updating the directory by means other than through the LDAP protocol,
 e.g., from inspection of server database files by database
 administrators.
 Sensitive data may be carried in almost any LDAP message, and its
 disclosure may be subject to privacy laws or other legal regulation
 in many countries.  Implementers should take appropriate measures to
 protect sensitive data from disclosure to unauthorized entities.
 A session on which the client has not established data integrity and
 privacy services (e.g., via StartTLS, IPsec, or a suitable SASL
 mechanism) is subject to man-in-the-middle attacks to view and modify
 information in transit.  Client and server implementers SHOULD take
 measures to protect sensitive data in the LDAP session from these
 attacks by using data protection services as discussed in this
 document.  Clients and servers should provide the ability to be
 configured to require these protections.  A resultCode of

Harrison Standards Track [Page 21] RFC 4513 LDAP Authentication Methods June 2006

 confidentialityRequired indicates that the server requires
 establishment of (stronger) data confidentiality protection in order
 to perform the requested operation.
 Access control should always be applied when reading sensitive
 information or updating directory information.
 Various security factors, including authentication and authorization
 information and data security services may change during the course
 of the LDAP session, or even during the performance of a particular
 operation.  Implementations should be robust in the handling of
 changing security factors.

6.2. StartTLS Security Considerations

 All security gained via use of the StartTLS operation is gained by
 the use of TLS itself.  The StartTLS operation, on its own, does not
 provide any additional security.
 The level of security provided through the use of TLS depends
 directly on both the quality of the TLS implementation used and the
 style of usage of that implementation.  Additionally, a man-in-the-
 middle attacker can remove the StartTLS extended operation from the
 'supportedExtension' attribute of the root DSE.  Both parties SHOULD
 independently ascertain and consent to the security level achieved
 once TLS is established and before beginning use of the TLS-
 protected session.  For example, the security level of the TLS layer
 might have been negotiated down to plaintext.
 Clients MUST either warn the user when the security level achieved
 does not provide an acceptable level of data confidentiality and/or
 data integrity protection, or be configurable to refuse to proceed
 without an acceptable level of security.
 As stated in Section 3.1.2, a server may use a local security policy
 to determine whether to successfully complete TLS negotiation.
 Information in the user's certificate that is originated or verified
 by the certification authority should be used by the policy
 administrator when configuring the identification and authorization
 policy.
 Server implementers SHOULD allow server administrators to elect
 whether and when data confidentiality and integrity are required, as
 well as elect whether authentication of the client during the TLS
 handshake is required.
 Implementers should be aware of and understand TLS security
 considerations as discussed in the TLS specification [RFC4346].

Harrison Standards Track [Page 22] RFC 4513 LDAP Authentication Methods June 2006

6.3. Bind Operation Security Considerations

 This section discusses several security considerations relevant to
 LDAP authentication via the Bind operation.

6.3.1. Unauthenticated Mechanism Security Considerations

 Operational experience shows that clients can (and frequently do)
 misuse the unauthenticated authentication mechanism of the simple
 Bind method (see Section 5.1.2).  For example, a client program might
 make a decision to grant access to non-directory information on the
 basis of successfully completing a Bind operation.  LDAP server
 implementations may return a success response to an unauthenticated
 Bind request.  This may erroneously leave the client with the
 impression that the server has successfully authenticated the
 identity represented by the distinguished name when in reality, an
 anonymous authorization state has been established.  Clients that use
 the results from a simple Bind operation to make authorization
 decisions should actively detect unauthenticated Bind requests (by
 verifying that the supplied password is not empty) and react
 appropriately.

6.3.2. Name/Password Mechanism Security Considerations

 The name/password authentication mechanism of the simple Bind method
 discloses the password to the server, which is an inherent security
 risk.  There are other mechanisms, such as SASL DIGEST-MD5
 [DIGEST-MD5], that do not disclose the password to the server.

6.3.3. Password-Related Security Considerations

 LDAP allows multi-valued password attributes.  In systems where
 entries are expected to have one and only one password,
 administrative controls should be provided to enforce this behavior.
 The use of clear text passwords and other unprotected authentication
 credentials is strongly discouraged over open networks when the
 underlying transport service cannot guarantee confidentiality.  LDAP
 implementations SHOULD NOT by default support authentication methods
 using clear text passwords and other unprotected authentication
 credentials unless the data on the session is protected using TLS or
 other data confidentiality and data integrity protection.
 The transmission of passwords in the clear -- typically for
 authentication or modification -- poses a significant security risk.
 This risk can be avoided by using SASL authentication [RFC4422]

Harrison Standards Track [Page 23] RFC 4513 LDAP Authentication Methods June 2006

 mechanisms that do not transmit passwords in the clear or by
 negotiating transport or session layer data confidentiality services
 before transmitting password values.
 To mitigate the security risks associated with the transfer of
 passwords, a server implementation that supports any password-based
 authentication mechanism that transmits passwords in the clear MUST
 support a policy mechanism that at the time of authentication or
 password modification, requires that:
       A TLS layer has been successfully installed.
       OR
       Some other data confidentiality mechanism that protects the
       password value from eavesdropping has been provided.
       OR
       The server returns a resultCode of confidentialityRequired for
       the operation (i.e., name/password Bind with password value,
       SASL Bind transmitting a password value in the clear, add or
       modify including a userPassword value, etc.), even if the
       password value is correct.
 Server implementations may also want to provide policy mechanisms to
 invalidate or otherwise protect accounts in situations where a server
 detects that a password for an account has been transmitted in the
 clear.

6.3.4. Hashed Password Security Considerations

 Some authentication mechanisms (e.g., DIGEST-MD5) transmit a hash of
 the password value that may be vulnerable to offline dictionary
 attacks.  Implementers should take care to protect such hashed
 password values during transmission using TLS or other
 confidentiality mechanisms.

6.4. SASL Security Considerations

 Until data integrity service is installed on an LDAP session, an
 attacker can modify the transmitted values of the
 'supportedSASLMechanisms' attribute response and thus downgrade the
 list of available SASL mechanisms to include only the least secure
 mechanism.  To detect this type of attack, the client may retrieve
 the SASL mechanisms the server makes available both before and after
 data integrity service is installed on an LDAP session.  If the
 client finds that the integrity-protected list (the list obtained

Harrison Standards Track [Page 24] RFC 4513 LDAP Authentication Methods June 2006

 after data integrity service was installed) contains a stronger
 mechanism than those in the previously obtained list, the client
 should assume the previously obtained list was modified by an
 attacker.  In this circumstance it is recommended that the client
 close the underlying transport connection and then reconnect to
 reestablish the session.

6.5. Related Security Considerations

 Additional security considerations relating to the various
 authentication methods and mechanisms discussed in this document
 apply and can be found in [RFC4422], [RFC4013], [RFC3454], and
 [RFC3629].

7. IANA Considerations

 The IANA has updated the LDAP Protocol Mechanism registry to indicate
 that this document and [RFC4511] provide the definitive technical
 specification for the StartTLS (1.3.6.1.4.1.1466.20037) extended
 operation.
 The IANA has updated the LDAP LDAPMessage types registry to indicate
 that this document and [RFC4511] provide the definitive technical
 specification for the bindRequest (0) and bindResponse (1) message
 types.
 The IANA has updated the LDAP Bind Authentication Method registry to
 indicate that this document and [RFC4511] provide the definitive
 technical specification for the simple (0) and sasl (3) bind
 authentication methods.
 The IANA has updated the LDAP authzid prefixes registry to indicate
 that this document provides the definitive technical specification
 for the dnAuthzId (dn:) and uAuthzId (u:) authzid prefixes.

8. Acknowledgements

 This document combines information originally contained in RFC 2251,
 RFC 2829, and RFC 2830.  RFC 2251 was a product of the Access,
 Searching, and Indexing of Directories (ASID) Working Group.  RFC
 2829 and RFC 2830 were products of the LDAP Extensions (LDAPEXT)
 Working Group.
 This document is a product of the IETF LDAP Revision (LDAPBIS)
 working group.

Harrison Standards Track [Page 25] RFC 4513 LDAP Authentication Methods June 2006

9. Normative References

 [RFC791]     Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.
 [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2460]    Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.
 [RFC3454]    Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.
 [RFC3490]    Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications
              (IDNA)", RFC 3490, March 2003.
 [RFC3629]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.
 [RFC4013]    Zeilenga, K., "SASLprep: Stringprep Profile for User
              Names and Passwords", RFC 4013, February 2005.
 [RFC4234]    Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.
 [RFC4346]    Dierks, T. and E. Rescorla, "The TLS Protocol Version
              1.1", RFC 4346, March 2006.
 [RFC4422]    Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              June 2006.
 [RFC4510]    Zeilenga, K., Ed., "Lightweight Directory Access
              Protocol (LDAP): Technical Specification Road Map", RFC
              4510, June 2006.
 [RFC4511]    Sermersheim, J., Ed., "Lightweight Directory Access
              Protocol (LDAP): The Protocol", RFC 4511, June 2006.
 [RFC4512]    Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Directory Information Models", RFC 4512, June
              2006.

Harrison Standards Track [Page 26] RFC 4513 LDAP Authentication Methods June 2006

 [RFC4514]    Zeilenga, K., Ed., "Lightweight Directory Access
              Protocol (LDAP): String Representation of Distinguished
              Names", RFC 4514, June 2006.
 [RFC4517]    Legg, S., Ed., "Lightweight Directory Access Protocol
              (LDAP): Syntaxes and Matching Rules", RFC 4517, June
              2006.
 [RFC4519]    Sciberras, A., Ed., "Lightweight Directory Access
              Protocol (LDAP): Schema for User Applications", RFC
              4519, June 2006.
 [RFC4520]    Zeilenga, K., "Internet Assigned Numbers Authority
              (IANA) Considerations for the Lightweight Directory
              Access Protocol (LDAP)", BCP 64, RFC 4520, June 2006.
 [Unicode]    The Unicode Consortium, "The Unicode Standard, Version
              3.2.0" is defined by "The Unicode Standard, Version 3.0"
              (Reading, MA, Addison-Wesley, 2000.  ISBN 0-201-61633-
              5), as amended by the "Unicode Standard Annex #27:
              Unicode 3.1" (http://www.unicode.org/reports/tr27/) and
              by the "Unicode Standard Annex #28: Unicode 3.2"
              (http://www.unicode.org/reports/tr28/).
 [X.501]      ITU-T Rec. X.501, "The Directory: Models", 1993.

10. Informative References

 [DIGEST-MD5] Leach, P., Newman, C., and A. Melnikov, "Using Digest
              Authentication as a SASL Mechanism", Work in Progress,
              March 2006.
 [PLAIN]      Zeilenga, K., "The Plain SASL Mechanism", Work in
              Progress, March 2005.
 [RFC2828]    Shirey, R., "Internet Security Glossary", FYI 36, RFC
              2828, May 2000.
 [RFC4301]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.
 [RFC4505]    Zeilenga, K., "The Anonymous SASL Mechanism", RFC 4505,
              June 2006.

Harrison Standards Track [Page 27] RFC 4513 LDAP Authentication Methods June 2006

Appendix A. Authentication and Authorization Concepts

 This appendix is non-normative.
 This appendix defines basic terms, concepts, and interrelationships
 regarding authentication, authorization, credentials, and identity.
 These concepts are used in describing how various security approaches
 are utilized in client authentication and authorization.

A.1. Access Control Policy

 An access control policy is a set of rules defining the protection of
 resources, generally in terms of the capabilities of persons or other
 entities accessing those resources.  Security objects and mechanisms,
 such as those described here, enable the expression of access control
 policies and their enforcement.

A.2. Access Control Factors

 A request, when it is being processed by a server, may be associated
 with a wide variety of security-related factors.  The server uses
 these factors to determine whether and how to process the request.
 These are called access control factors (ACFs).  They might include
 source IP address, encryption strength, the type of operation being
 requested, time of day, etc..  Some factors may be specific to the
 request itself; others may be associated with the transport
 connection via which the request is transmitted; and others (e.g.,
 time of day) may be "environmental".
 Access control policies are expressed in terms of access control
 factors; for example, "a request having ACFs i,j,k can perform
 operation Y on resource Z".  The set of ACFs that a server makes
 available for such expressions is implementation specific.

A.3. Authentication, Credentials, Identity

 Authentication credentials are the evidence supplied by one party to
 another, asserting the identity of the supplying party (e.g., a user)
 who is attempting to establish a new authorization state with the
 other party (typically a server).  Authentication is the process of
 generating, transmitting, and verifying these credentials and thus
 the identity they assert.  An authentication identity is the name
 presented in a credential.
 There are many forms of authentication credentials.  The form used
 depends upon the particular authentication mechanism negotiated by
 the parties.  X.509 certificates, Kerberos tickets, and simple
 identity and password pairs are all examples of authentication

Harrison Standards Track [Page 28] RFC 4513 LDAP Authentication Methods June 2006

 credential forms.  Note that an authentication mechanism may
 constrain the form of authentication identities used with it.

A.4. Authorization Identity

 An authorization identity is one kind of access control factor.  It
 is the name of the user or other entity that requests that operations
 be performed.  Access control policies are often expressed in terms
 of authorization identities; for example, "entity X can perform
 operation Y on resource Z".
 The authorization identity of an LDAP session is often semantically
 the same as the authentication identity presented by the client, but
 it may be different.  SASL allows clients to specify an authorization
 identity distinct from the authentication identity asserted by the
 client's credentials.  This permits agents such as proxy servers to
 authenticate using their own credentials, yet request the access
 privileges of the identity for which they are proxying [RFC4422].
 Also, the form of authentication identity supplied by a service like
 TLS may not correspond to the authorization identities used to
 express a server's access control policy, thus requiring a server-
 specific mapping to be done.  The method by which a server composes
 and validates an authorization identity from the authentication
 credentials supplied by a client is implementation specific.

Appendix B. Summary of Changes

 This appendix is non-normative.
 This appendix summarizes substantive changes made to RFC 2251, RFC
 2829 and RFC 2830.  In addition to the specific changes detailed
 below, the reader of this document should be aware that numerous
 general editorial changes have been made to the original content from
 the source documents.  These changes include the following:
  1. The material originally found in RFC 2251 Sections 4.2.1 and 4.2.2,

RFC 2829 (all sections except Sections 2 and 4), and RFC 2830 was

   combined into a single document.
  1. The combined material was substantially reorganized and edited to

group related subjects, improve the document flow, and clarify

   intent.
  1. Changes were made throughout the text to align with definitions of

LDAP protocol layers and IETF security terminology.

Harrison Standards Track [Page 29] RFC 4513 LDAP Authentication Methods June 2006

  1. Substantial updates and additions were made to security

considerations from both documents based on current operational

   experience.

B.1. Changes Made to RFC 2251

 This section summarizes the substantive changes made to Sections
 4.2.1 and 4.2.2 of RFC 2251 by this document.  Additional substantive
 changes to Section 4.2.1 of RFC 2251 are also documented in
 [RFC4511].

B.1.1. Section 4.2.1 ("Sequencing of the Bind Request")

  1. Paragraph 1: Removed the sentence, "If at any stage the client

wishes to abort the bind process it MAY unbind and then drop the

   underlying connection".  The Unbind operation still permits this
   behavior, but it is not documented explicitly.
  1. Clarified that the session is moved to an anonymous state upon

receipt of the BindRequest PDU and that it is only moved to a non-

   anonymous state if and when the Bind request is successful.

B.1.2. Section 4.2.2 ("Authentication and Other Security Services")

  1. RFC 2251 states that anonymous authentication MUST be performed

using the simple bind method. This specification defines the

   anonymous authentication mechanism of the simple bind method and
   requires all conforming implementations to support it.  Other
   authentication mechanisms producing anonymous authentication and
   authorization state may also be implemented and used by conforming
   implementations.

B.2. Changes Made to RFC 2829

 This section summarizes the substantive changes made to RFC 2829.

B.2.1. Section 4 ("Required security mechanisms")

  1. The name/password authentication mechanism (see Section B.2.5

below) protected by TLS replaces the SASL DIGEST-MD5 mechanism as

   LDAP's mandatory-to-implement password-based authentication
   mechanism.  Implementations are encouraged to continue supporting
   SASL DIGEST-MD5 [DIGEST-MD5].

Harrison Standards Track [Page 30] RFC 4513 LDAP Authentication Methods June 2006

B.2.2. Section 5.1 ("Anonymous authentication procedure")

  1. Clarified that anonymous authentication involves a name value of

zero length and a password value of zero length. The

   unauthenticated authentication mechanism was added to handle simple
   Bind requests involving a name value with a non-zero length and a
   password value of zero length.

B.2.3. Section 6 ("Password-based authentication")

  1. See Section B.2.1.

B.2.4. Section 6.1 ("Digest authentication")

  1. As the SASL-DIGEST-MD5 mechanism is no longer mandatory to

implement, this section is now historical and was not included in

   this document.  RFC 2829, Section 6.1, continues to document the
   SASL DIGEST-MD5 authentication mechanism.

B.2.5. Section 6.2 ("'simple' authentication choice under TLS

      encryption")
  1. Renamed the "simple" authentication mechanism to the name/password

authentication mechanism to better describe it.

  1. The use of TLS was generalized to align with definitions of LDAP

protocol layers. TLS establishment is now discussed as an

   independent subject and is generalized for use with all
   authentication mechanisms and other security layers.
  1. Removed the implication that the userPassword attribute is the sole

location for storage of password values to be used in

   authentication.  There is no longer any implied requirement for how
   or where passwords are stored at the server for use in
   authentication.

B.2.6. Section 6.3 ("Other authentication choices with TLS")

  1. See Section B.2.5.

B.2.7. Section 7.1 ("Certificate-based authentication with TLS")

  1. See Section B.2.5.

Harrison Standards Track [Page 31] RFC 4513 LDAP Authentication Methods June 2006

B.2.8. Section 8 ("Other mechanisms")

  1. All SASL authentication mechanisms are explicitly allowed within

LDAP. Specifically, this means the SASL ANONYMOUS and SASL PLAIN

   mechanisms are no longer precluded from use within LDAP.

B.2.9. Section 9 ("Authorization Identity")

  1. Specified matching rules for dnAuthzId and uAuthzId values. In

particular, the DN value in the dnAuthzId form must be matched

   using DN matching rules, and the uAuthzId value MUST be prepared
   using SASLprep rules before being compared octet-wise.
  1. Clarified that uAuthzId values should not be assumed to be globally

unique.

B.2.10. Section 10 ("TLS Ciphersuites")

  1. TLS ciphersuite recommendations are no longer included in this

specification. Implementations must now support the

   TLS_RSA_WITH_3DES_EDE_CBC_SHA ciphersuite and should continue to
   support the TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA ciphersuite.
  1. Clarified that anonymous authentication involves a name value of

zero length and a password value of zero length. The

   unauthenticated authentication mechanism was added to handle simple
   Bind requests involving a name value with a non-zero length and a
   password value of zero length.

B.3. Changes Made to RFC 2830

 This section summarizes the substantive changes made to Sections 3
 and 5 of RFC 2830.  Readers should consult [RFC4511] for summaries of
 changes to other sections.

B.3.1. Section 3.6 ("Server Identity Check")

  1. Substantially updated the server identity check algorithm to ensure

that it is complete and robust. In particular, the use of all

   relevant values in the subjectAltName and the subjectName fields
   are covered by the algorithm and matching rules are specified for
   each type of value.  Mapped (derived) forms of the server identity
   may now be used when the mapping is performed in a secure fashion.

Harrison Standards Track [Page 32] RFC 4513 LDAP Authentication Methods June 2006

B.3.2. Section 3.7 ("Refresh of Server Capabilities Information")

  1. Clients are no longer required to always refresh information about

server capabilities following TLS establishment. This is to allow

   for situations where this information was obtained through a secure
   mechanism.

B.3.3. Section 5 ("Effects of TLS on a Client's Authorization

      Identity")
  1. Establishing a TLS layer on an LDAP session may now cause the

authorization state of the LDAP session to change.

B.3.4. Section 5.2 ("TLS Connection Closure Effects")

  1. Closing a TLS layer on an LDAP session changes the authentication

and authorization state of the LDAP session based on local policy.

   Specifically, this means that implementations are not required to
   change the authentication and authorization states to anonymous
   upon TLS closure.
  1. Replaced references to RFC 2401 with RFC 4301.

Author's Address

 Roger Harrison
 Novell, Inc.
 1800 S.  Novell Place
 Provo, UT 84606
 USA
 Phone: +1 801 861 2642
 EMail: roger_harrison@novell.com

Harrison Standards Track [Page 33] RFC 4513 LDAP Authentication Methods June 2006

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Harrison Standards Track [Page 34]

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