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.
AbstractThis document specifies particular combinations of security mechanisms which are required and recommended in LDAP  implementations.
(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 mechanisms: (1) Client authentication by means of the SASL  mechanism set, possibly backed by the TLS credentials exchange mechanism, (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 mechanism. 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.
section 4.2 of ). 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.
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.
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 , 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 , 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).
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 DSE. 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] to negotiate the use of TLS security . 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.
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. section 2.1 of , 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 . 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 . 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 . Support for subsequent authentication is OPTIONAL in clients and servers.
6]. The client will use the Start TLS operation  to negotiate the use of TLS security  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. section 7.1.
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  to negotiate the use of TLS security  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.
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 ), 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. 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 unique. Additional authorization identity schemes MAY be defined in future versions of this document. 6] MUST NOT be used for confidentiality protection of passwords or data: TLS_NULL_WITH_NULL_NULL TLS_RSA_WITH_NULL_MD5 TLS_RSA_WITH_NULL_SHA The following ciphersuites defined in  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: TLS_RSA_EXPORT_WITH_RC4_40_MD5 TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 TLS_RSA_EXPORT_WITH_DES40_CBC_SHA TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA 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:
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 TLS_DH_anon_WITH_RC4_128_MD5 TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA TLS_DH_anon_WITH_DES_CBC_SHA TLS_DH_anon_WITH_3DES_EDE_CBC_SHA A client or server that supports TLS MUST support at least TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA. 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. 2],  and .
 Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access Protocol (v3)", RFC 2251, December 1997.  Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Leach, P. and C. Newman, "Using Digest Authentication as a SASL Mechanism", RFC 2831, May 2000.  Hodges, J., Morgan, R. and M. Wahl, "Lightweight Directory Access Protocol (v3): Extension for Transport Layer Security", RFC 2830, May 2000.  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999.  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997.  Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998.
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