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RFC 2829 - Authentication Methods for LDAP

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Network Working Group                                            M. Wahl
Request for Comments: 2829                        Sun Microsystems, Inc.
Category: Standards Track                                  H. Alvestrand
                                                             EDB Maxware
                                                               J. Hodges
                                                             Oblix, Inc.
                                                               R. Morgan
                                                University of Washington
                                                                May 2000

                    Authentication Methods for LDAP

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 (2000).  All Rights Reserved.


   This document specifies particular combinations of security
   mechanisms which are required and recommended in LDAP [1]

1. Introduction

   LDAP version 3 is a powerful access protocol for directories.

   It offers means of searching, fetching and manipulating directory
   content, and ways to access a rich set of security functions.

   In order to function for the best of the Internet, it is vital that
   these security functions be interoperable; therefore there has to be
   a minimum subset of security functions that is common to all
   implementations that claim LDAPv3 conformance.

   Basic threats to an LDAP directory service include:

      (1)   Unauthorized access to data via data-fetching operations,

      (2)   Unauthorized access to reusable client authentication
            information by monitoring others' access,

      (3)   Unauthorized access to data by monitoring others' access,

      (4)   Unauthorized modification of data,

      (5)   Unauthorized modification of configuration,

      (6)   Unauthorized or excessive use of resources (denial of
            service), and

      (7)   Spoofing of directory: Tricking a 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 connection.

   Threats (1), (4), (5) and (6) are due to hostile clients.  Threats
   (2), (3) and (7) are due to hostile agents on the path between client
   and server, or posing as a server.

   The LDAP protocol suite can be protected with the following security

      (1)   Client authentication by means of the SASL [2] mechanism
            set, possibly backed by the TLS credentials exchange

      (2)   Client authorization by means of access control based on the
            requestor's authenticated identity,

      (3)   Data integrity protection by means of the TLS protocol or
            data-integrity SASL mechanisms,

      (4)   Protection against snooping by means of the TLS protocol or
            data-encrypting SASL mechanisms,

      (5)   Resource limitation by means of administrative limits on
            service controls, and

      (6)   Server authentication by means of the TLS protocol or SASL

   At the moment, imposition of access controls is done by means outside
   the scope of the LDAP protocol.

   In this document, the term "user" represents any application which is
   an LDAP client using the directory to retrieve or store information.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

2.  Example deployment scenarios

   The following scenarios are typical for LDAP directories on the
   Internet, and have different security requirements. (In the
   following, "sensitive" means data that will cause real damage to the
   owner if revealed; there may be data that is protected but not
   sensitive).  This is not intended to be a comprehensive list, other
   scenarios are possible, especially on physically protected networks.

      (1)   A read-only directory, containing no sensitive data,
            accessible to "anyone", and TCP connection hijacking or IP
            spoofing is not a problem.  This directory requires no
            security functions except administrative service limits.

      (2)   A read-only directory containing no sensitive data; read
            access is granted based on identity.  TCP connection
            hijacking is not currently a problem. This scenario requires
            a secure authentication function.

      (3)   A read-only directory containing no sensitive data; and the
            client needs to ensure that the directory data is
            authenticated by the server and not modified while being
            returned from the server.

      (4)   A read-write directory, containing no sensitive data; read
            access is available to "anyone", update access to properly
            authorized persons.  TCP connection hijacking is not
            currently a problem.  This scenario requires a secure
            authentication function.

      (5)   A directory containing sensitive data.  This scenario
            requires session confidentiality protection AND secure

3.  Authentication and Authorization:  Definitions and Concepts

   This section 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.

3.1.  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.  A common expression of an access
   control policy is an access control list.  Security objects and
   mechanisms, such as those described here, enable the expression of
   access control policies and their enforcement.  Access control
   policies are typically expressed in terms of access control
   attributes as described below.

3.2.  Access Control Factors

   A request, when it is being processed by a server, may be associated
   with a wide variety of security-related factors (section 4.2 of [1]).
   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
   connection via which the request is transmitted, others (e.g. time of
   day) may be "environmental".

   Access control policies are expressed in terms of access control
   factors.  E.g., 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.

3.3.  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 an association 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

   There are many forms of authentication credentials -- the form used
   depends upon the particular authentication mechanism negotiated by
   the parties.  For example: X.509 certificates, Kerberos tickets,
   simple identity and password pairs.  Note that an authentication
   mechanism may constrain the form of authentication identities used
   with it.

3.4.  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; e.g., entity X can perform operation Y
   on resource Z.

   The authorization identity bound to an association is often exactly
   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 [2].  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, 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.

4. Required security mechanisms

   It is clear that allowing any implementation, faced with the above
   requirements, to pick and choose among the possible alternatives is
   not a strategy that is likely to lead to interoperability. In the
   absence of mandates, clients will be written that do not support any
   security function supported by the server, or worse, support only
   mechanisms like cleartext passwords that provide clearly inadequate

   Active intermediary attacks are the most difficult for an attacker to
   perform, and for an implementation to protect against.  Methods that
   protect only against hostile client and passive eavesdropping attacks
   are useful in situations where the cost of protection against active
   intermediary attacks is not justified based on the perceived risk of
   active intermediary attacks.

   Given the presence of the Directory, there is a strong desire to see
   mechanisms where identities take the form of a Distinguished Name and
   authentication data can be stored in the directory; this means that
   either this data is useless for faking authentication (like the Unix
   "/etc/passwd" file format used to be), or its content is never passed
   across the wire unprotected - that is, it's either updated outside
   the protocol or it is only updated in sessions well protected against
   snooping.  It is also desirable to allow authentication methods to

   carry authorization identities based on existing forms of user
   identities for backwards compatibility with non-LDAP-based
   authentication services.

   Therefore, the following implementation conformance requirements are
   in place:

      (1)   For a read-only, public directory, anonymous authentication,
            described in section 5, can be used.

      (2)   Implementations providing password-based authenticated
            access MUST support authentication using the DIGEST-MD5 SASL
            mechanism [4], as described in section 6.1.  This provides
            client authentication with protection against passive
            eavesdropping attacks, but does not provide protection
            against active intermediary attacks.

      (3)   For a directory needing session protection and
            authentication, the Start TLS extended operation [5], and
            either the simple authentication choice or the SASL EXTERNAL
            mechanism, are to be used together.  Implementations SHOULD
            support authentication with a password as described in
            section 6.2, and SHOULD support authentication with a
            certificate as described in section 7.1.  Together, these
            can provide integrity and disclosure protection of
            transmitted data, and authentication of client and server,
            including protection against active intermediary attacks.

   If TLS is negotiated, the client MUST discard all information about
   the server fetched prior to the TLS negotiation.  In particular, the
   value of supportedSASLMechanisms MAY be different after TLS has been
   negotiated (specifically, the EXTERNAL mechanism or the proposed
   PLAIN mechanism are likely to only be listed after a TLS negotiation
   has been performed).

   If a SASL security layer is negotiated, the client MUST discard all
   information about the server fetched prior to SASL.  In particular,
   if the client is configured to support multiple SASL mechanisms, it
   SHOULD fetch supportedSASLMechanisms both before and after the SASL
   security layer is negotiated and verify that the value has not
   changed after the SASL security layer was negotiated.  This detects
   active attacks which remove supported SASL mechanisms from the
   supportedSASLMechanisms list, and allows the client to ensure that it
   is using the best mechanism supported by both client and server
   (additionally, this is a SHOULD to allow for environments where the
   supported SASL mechanisms list is provided to the client through a
   different trusted source, e.g. as part of a digitally signed object).

5. Anonymous authentication

   Directory operations which modify entries or access protected
   attributes or entries generally require client authentication.
   Clients which do not intend to perform any of these operations
   typically use anonymous authentication.

   LDAP implementations MUST support anonymous authentication, as
   defined in section 5.1.

   LDAP implementations MAY support anonymous authentication with TLS,
   as defined in section 5.2.

   While there MAY be access control restrictions to prevent access to
   directory entries, an LDAP server SHOULD allow an anonymously-bound
   client to retrieve the supportedSASLMechanisms attribute of the root

   An LDAP server MAY use other information about the client provided by
   the lower layers or external means to grant or deny access even to
   anonymously authenticated clients.

5.1. Anonymous authentication procedure

   An LDAP client which has not successfully completed a bind operation
   on a connection is anonymously authenticated.

   An LDAP client MAY also specify anonymous authentication in a bind
   request by using a zero-length OCTET STRING with the simple
   authentication choice.

5.2. Anonymous authentication and TLS

   An LDAP client MAY use the Start TLS operation [5] to negotiate the
   use of TLS security [6].  If the client has not bound beforehand,
   then until the client uses the EXTERNAL SASL mechanism to negotiate
   the recognition of the client's certificate, the client is
   anonymously authenticated.

   Recommendations on TLS ciphersuites are given in section 10.

   An LDAP server which requests that clients provide their certificate
   during TLS negotiation MAY use a local security policy to determine
   whether to successfully complete TLS negotiation if the client did
   not present a certificate which could be validated.

6. Password-based authentication

   LDAP implementations MUST support authentication with a password
   using the DIGEST-MD5 SASL mechanism for password protection, as
   defined in section 6.1.

   LDAP implementations SHOULD support authentication with the "simple"
   password choice when the connection is protected against
   eavesdropping using TLS, as defined in section 6.2.

6.1. Digest authentication

   An LDAP client MAY determine whether the server supports this
   mechanism by performing a search request on the root DSE, requesting
   the supportedSASLMechanisms attribute, and checking whether the
   string "DIGEST-MD5" is present as a value of this attribute.

   In the first stage of authentication, when the client is performing
   an "initial authentication" as defined in section 2.1 of [4], the
   client sends a bind request in which the version number is 3, the
   authentication choice is sasl, the sasl mechanism name is "DIGEST-
   MD5", and the credentials are absent.  The client then waits for a
   response from the server to this request.

   The server will respond with a bind response in which the resultCode
   is saslBindInProgress, and the serverSaslCreds field is present.  The
   contents of this field is a string defined by "digest-challenge" in
   section 2.1.1 of [4].  The server SHOULD include a realm indication
   and MUST indicate support for UTF-8.

   The client will send a bind request with a distinct message id, in
   which the version number is 3, the authentication choice is sasl, the
   sasl mechanism name is "DIGEST-MD5", and the credentials contain the
   string defined by "digest-response" in section 2.1.2 of [4].  The
   serv-type is "ldap".

   The server will respond with a bind response in which the resultCode
   is either success, or an error indication.  If the authentication is
   successful and the server does not support subsequent authentication,
   then the credentials field is absent.  If the authentication is
   successful and the server supports subsequent authentication, then
   the credentials field contains the string defined by "response-auth"
   in section 2.1.3 of [4].   Support for subsequent authentication is
   OPTIONAL in clients and servers.

6.2. "simple" authentication choice under TLS encryption

   A user who has a directory entry containing a userPassword attribute
   MAY authenticate to the directory by performing a simple password
   bind sequence following the negotiation of a TLS ciphersuite
   providing connection confidentiality [6].

   The client will use the Start TLS operation [5] to negotiate the use
   of TLS security [6] on the connection to the LDAP server.  The client
   need not have bound to the directory beforehand.

   For this authentication procedure to be successful, the client and
   server MUST negotiate a ciphersuite which contains a bulk encryption
   algorithm of appropriate strength.  Recommendations on cipher suites
   are given in section 10.

   Following the successful completion of TLS negotiation, the client
   MUST send an LDAP bind request with the version number of 3, the name
   field containing the name of the user's entry, and the "simple"
   authentication choice, containing a password.

   The server will, for each value of the userPassword attribute in the
   named user's entry, compare these for case-sensitive equality with
   the client's presented password.  If there is a match, then the
   server will respond with resultCode success, otherwise the server
   will respond with resultCode invalidCredentials.

6.3. Other authentication choices with TLS

   It is also possible, following the negotiation of TLS, to perform a
   SASL authentication which does not involve the exchange of plaintext
   reusable passwords.  In this case the client and server need not
   negotiate a ciphersuite which provides confidentiality if the only
   service required is data integrity.

7. Certificate-based authentication

   LDAP implementations SHOULD support authentication via a client
   certificate in TLS, as defined in section 7.1.

7.1. Certificate-based authentication with TLS

   A user who has a public/private key pair in which the public key has
   been signed by a Certification Authority may use this key pair to
   authenticate to the directory server if the user's certificate is
   requested by the server.  The user's certificate subject field SHOULD
   be the name of the user's directory entry, and the Certification
   Authority must be sufficiently trusted by the directory server to

   have issued the certificate in order that the server can process the
   certificate.  The means by which servers validate certificate paths
   is outside the scope of this document.

   A server MAY support mappings for certificates in which the subject
   field name is different from the name of the user's directory entry.
   A server which supports mappings of names MUST be capable of being
   configured to support certificates for which no mapping is required.

   The client will use the Start TLS operation [5] to negotiate the use
   of TLS security [6] on the connection to the LDAP server.  The client
   need not have bound to the directory beforehand.

   In the TLS negotiation, the server MUST request a certificate.  The
   client will provide its certificate to the server, and MUST perform a
   private key-based encryption, proving it has the private key
   associated with the certificate.

   As deployments will require protection of sensitive data in transit,
   the client and server MUST negotiate a ciphersuite which contains a
   bulk encryption algorithm of appropriate strength.  Recommendations
   of cipher suites are given in section 10.

   The server MUST verify that the client's certificate is valid. The
   server will normally check that the certificate is issued by a known
   CA, and that none of the certificates on the client's certificate
   chain are invalid or revoked.  There are several procedures by which
   the server can perform these checks.

   Following the successful completion of TLS negotiation, the client
   will send an LDAP bind request with the SASL "EXTERNAL" mechanism.

8. Other mechanisms

   The LDAP "simple" authentication choice is not suitable for
   authentication on the Internet where there is no network or transport
   layer confidentiality.

   As LDAP includes native anonymous and plaintext authentication
   methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not used
   with LDAP.  If an authorization identity of a form different from a
   DN is requested by the client, a mechanism that protects the password
   in transit SHOULD be used.

   The following SASL-based mechanisms are not considered in this
   document: KERBEROS_V4, GSSAPI and SKEY.

   The "EXTERNAL" SASL mechanism can be used to request the LDAP server
   make use of security credentials exchanged by a lower layer. If a TLS
   session has not been established between the client and server prior
   to making the SASL EXTERNAL Bind request and there is no other
   external source of authentication credentials (e.g.  IP-level
   security [8]), or if, during the process of establishing the TLS
   session, the server did not request the client's authentication
   credentials, the SASL EXTERNAL bind MUST fail with a result code of
   inappropriateAuthentication.  Any client authentication and
   authorization state of the LDAP association is lost, so the LDAP
   association is in an anonymous state after the failure.

9. Authorization Identity

   The authorization identity is carried as part of the SASL credentials
   field in the LDAP Bind request and response.

   When the "EXTERNAL" mechanism is being negotiated, if the credentials
   field is present, it contains an authorization identity of the
   authzId form described below.

   Other mechanisms define the location of the authorization identity in
   the credentials field.

   The authorization identity is a string in the UTF-8 character set,
   corresponding to the following ABNF [7]:

   ; Specific predefined authorization (authz) id schemes are
   ; defined below -- new schemes may be defined in the future.

   authzId    = dnAuthzId / uAuthzId

   ; distinguished-name-based authz id.
   dnAuthzId  = "dn:" dn
   dn         = utf8string    ; with syntax defined in RFC 2253

   ; unspecified userid, UTF-8 encoded.
   uAuthzId   = "u:" userid
   userid     = utf8string    ; syntax unspecified

   A utf8string is defined to be the UTF-8 encoding of one or more ISO
   10646 characters.

   All servers which support the storage of authentication credentials,
   such as passwords or certificates, in the directory MUST support the
   dnAuthzId choice.

   The uAuthzId choice allows for compatibility with client applications
   which wish to authenticate to a local directory but do not know their
   own Distinguished Name or have a directory entry.  The format of the
   string is defined as only a sequence of UTF-8 encoded ISO 10646
   characters, and further interpretation is subject to prior agreement
   between the client and server.

   For example, the userid could identify a user of a specific directory
   service, or be a login name or the local-part of an RFC 822 email
   address. In general a uAuthzId MUST NOT be assumed to be globally

   Additional authorization identity schemes MAY be defined in future
   versions of this document.

10. TLS Ciphersuites

   The following ciphersuites defined in [6] MUST NOT be used for
   confidentiality protection of passwords or data:


   The following ciphersuites defined in [6] can be cracked easily (less
   than a week of CPU time on a standard CPU in 1997).  The client and
   server SHOULD carefully consider the value of the password or data
   being protected before using these ciphersuites:


   The following ciphersuites are vulnerable to man-in-the-middle
   attacks, and 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 tolerable:


   A client or server that supports TLS MUST support at least

11. SASL service name for LDAP

   For use with SASL [2], a protocol must specify a service name to be
   used with various SASL mechanisms, such as GSSAPI.  For LDAP, the
   service name is "ldap", which has been registered with the IANA as a
   GSSAPI service name.

12. Security Considerations

   Security issues are discussed throughout this memo; the
   (unsurprising) conclusion is that mandatory security is important,
   and that session encryption is required when snooping is a problem.

   Servers are encouraged to prevent modifications by anonymous users.
   Servers may also wish to minimize denial of service attacks by timing
   out idle connections, and returning the unwillingToPerform result
   code rather than performing computationally expensive operations
   requested by unauthorized clients.

   A connection on which the client has not performed the Start TLS
   operation or negotiated a suitable SASL mechanism for connection
   integrity and encryption services is subject to man-in-the-middle
   attacks to view and modify information in transit.

   Additional security considerations relating to the EXTERNAL mechanism
   to negotiate TLS can be found in [2], [5] and [6].

13. Acknowledgements

   This document is a product of the LDAPEXT Working Group of the IETF.
   The contributions of its members is greatly appreciated.

14. Bibliography

   [1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
       Protocol (v3)", RFC 2251, December 1997.

   [2] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC
       2222, October 1997.

   [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

   [4] Leach, P. and C. Newman, "Using Digest Authentication as a SASL
       Mechanism", RFC 2831, May 2000.

   [5] Hodges, J., Morgan, R. and M. Wahl, "Lightweight Directory Access
       Protocol (v3): Extension for Transport Layer Security", RFC 2830,
       May 2000.

   [6] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
       2246, January 1999.

   [7] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
       Specifications: ABNF", RFC 2234, November 1997.

   [8] Kent, S. and R. Atkinson, "Security Architecture for the Internet
       Protocol", RFC 2401, November 1998.

15. Authors' Addresses

   Mark Wahl
   Sun Microsystems, Inc.
   8911 Capital of Texas Hwy #4140
   Austin TX 78759

   EMail: M.Wahl@innosoft.com

   Harald Tveit Alvestrand
   EDB Maxware
   N-7462 Trondheim, Norway

   Phone: +47 73 54 57 97
   EMail: Harald@Alvestrand.no

   Jeff Hodges
   Oblix, Inc.
   18922 Forge Drive
   Cupertino, CA 95014

   Phone: +1-408-861-6656
   EMail: JHodges@oblix.com

   RL "Bob" Morgan
   Computing and Communications
   University of Washington
   Seattle, WA 98105

   Phone: +1-206-221-3307
   EMail: rlmorgan@washington.edu

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