rfc4513.txt

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:

      - when TLS is currently established on the session,
      - when a multi-stage SASL negotiation is in progress on the
        session, or
      - 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:

      - 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.

      - 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.

      - 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.

      - 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:

      - An anonymous authentication mechanism (Section 5.1.1).

      - An unauthenticated authentication mechanism (Section 5.1.2).

      - 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:

      - The version is 3.
      - The AuthenticationChoice is sasl.
      - The mechanism element of the SaslCredentials sequence contains
        the value of the desired SASL mechanism.
      - 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).