RFC 2829

Authentication Methods for LDAP

📅 published May 2000, from ietf

RFC 2829: Authentication Methods for LDAP 🎀

okay bestie, this one is a Standards Track doc from May 2000 (💅 it's an official Internet standard). It's from the Network Working Group, and four people wrote it:

  • M. Wahl, Sun Microsystems, Inc.
  • H. Alvestrand, EDB Maxware
  • J. Hodges, Oblix, Inc.
  • R. Morgan, University of Washington

It doesn't obsolete or update anything, since its header has no Obsoletes or Updates lines. 🦋

Status of This Memo ✨

so this doc is an Internet standards track protocol for the Internet community, and it's basically asking for discussion and suggestions for improvements 💖 If you want to know where it stands right now, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited, so share it wherever 🌸

Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract 🔥

the whole point is pinning down particular combinations of security mechanisms that are required and recommended in LDAP [1] implementations 🫶

1. Introduction 🧚

LDAP version 3 is a seriously powerful access protocol for directories 👑

It lets you search, fetch and manipulate directory content, and it gives you ways to reach a rich set of security functions ✨

Here's the thing: for the Internet to work at its best, these security functions have to be interoperable 😭 So there needs to be a minimum subset of security functions that every implementation claiming LDAPv3 conformance has in common.

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 come from hostile clients. Threats 2, 3 and 7 come from hostile agents on the path between client and server, or from something posing as a server. Not me being like "who would do that" 💀

The LDAP protocol suite can be protected with the following security mechanisms:

  1. Client authentication by means of the SASL [2] 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, imposing access controls is done by means outside the scope of the LDAP protocol 🙈

In this document, the term "user" means any application that's an LDAP client using the directory to retrieve or store information 🍓

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [3]. This is the part where the caps lock matters, so pay attention 🫶

2. Example deployment scenarios 🌸

These scenarios are typical for LDAP directories on the Internet, and each one has different security requirements. (Quick note: "sensitive" here means data that will cause real damage to the owner if it's revealed. There might be data that's protected but not sensitive.) This list isn't meant to be comprehensive, and other scenarios are possible, especially on physically protected networks 🦋

  1. A read-only directory, containing no sensitive data, accessible to "anyone", where 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, where 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, where the client needs to make sure 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, where read access is available to "anyone" and update access goes 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 authentication. ⭐

3. Authentication and Authorization: Definitions and Concepts 💜

this section is the vocab lesson 📚 It defines the basic terms, concepts, and how they relate to each other when it comes to authentication, authorization, credentials, and identity. The rest of the doc uses all of this to explain how the different security approaches get used for client authentication and authorization.

3.1. Access Control Policy 🔐

An access control policy is a set of rules that protects resources, usually in terms of what persons or other entities accessing those resources are able to do. A common way to write one down is an access control list. Security objects and mechanisms, like the ones described here, let you express access control policies and enforce them. Access control policies are usually written in terms of access control attributes, which are described below.

3.2. Access Control Factors 🧁

When a server processes a request, that request can be linked to a whole bunch of security-related factors (section 4.2 of [1]). The server uses these factors to decide whether to process the request and how. These are called access control factors (ACFs). They might include source IP address, encryption strength, the type of operation being requested, time of day, and so on. Some factors belong to the request itself, some belong to the connection the request is sent over, and some (like time of day) are "environmental" 🌤️

Access control policies get expressed in terms of access control factors. For example, a request having ACFs i,j,k can perform operation Y on resource Z. Which ACFs a server makes available for these expressions is implementation-specific 💅

3.3. Authentication, Credentials, Identity 🪪

Authentication credentials are the evidence one party gives another to prove the identity of the party supplying it (for example, a user) who's trying to set up an association with the other party (usually a server). Authentication is the whole process of generating, transmitting, and verifying those credentials, and therefore verifying the identity they claim. An authentication identity is the name presented in a credential ✨

There are lots of forms of authentication credentials, and the form you get depends on the particular authentication mechanism the parties negotiated. Examples include X.509 certificates, Kerberos tickets, and simple identity and password pairs. One thing to note: 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's the name of the user or other entity that requests that operations be performed. Access control policies are often written in terms of authorization identities, like "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 the client presents, but it doesn't have to be. SASL lets clients specify an authorization identity that's different from the authentication identity asserted by their credentials. This lets agents like proxy servers authenticate with their own credentials and still request the access privileges of the identity they're proxying for [2]. Also, the form of authentication identity supplied by a service like TLS may not match the authorization identities used to express a server's access control policy, so a server-specific mapping has to be done. How a server composes and validates an authorization identity from the authentication credentials a client supplies is implementation-specific 🧚

4. Required security mechanisms ✨

okay so this is where it gets real, bestie 💅 If every implementation just gets to pick and choose from the security options, nobody's gonna be able to talk to each other. Clients get written that don't support any security function the server offers, or worse, they only do cleartext passwords, and that is literally giving no protection 😭 So we need some mandates.

Active intermediary attacks are the hardest ones for an attacker to pull off, AND the hardest for an implementation to defend against 🕵️ Methods that only protect against hostile clients and passive eavesdropping are still useful, though, in situations where protecting against active intermediary attacks isn't worth the cost based on how risky those attacks seem.

Since the Directory is a big deal here, there's a strong want for mechanisms where identities look like a Distinguished Name and authentication data can be stored in the directory. That means one of two things. Either the data is useless for faking authentication (like the old Unix "/etc/passwd" file format used to be), or its content is never passed across the wire unprotected. That second option means it's either updated outside the protocol, or only updated in sessions that are well protected against snooping. It's also nice to let authentication methods carry authorization identities based on existing forms of user identities, for backwards compatibility with non-LDAP-based authentication services 🫶

So, the implementation conformance requirements are:

  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 make sure it's 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 don't 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 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 🌸

5.1. Anonymous authentication procedure

An LDAP client which has not successfully completed a bind operation on a connection is anonymously authenticated. Main character energy for the no-login era 💖

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 ✨

Okay bestie, so if a user has a public/private key pair whose public key was signed by a Certification Authority, they can use that pair to authenticate to the directory server, as long as the server requests the user's certificate. The user's certificate subject field SHOULD be the name of the user's directory entry. The Certification Authority must be trusted enough by the directory server to have issued the certificate, so the server can process it. How servers validate certificate paths is outside the scope of this document. 💅

A server MAY support mappings for certificates where the subject field name is different from the name of the user's directory entry. A server that supports name mappings MUST be capable of being configured to support certificates where 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. 🌸

During the TLS negotiation, the server MUST request a certificate. The client will give its certificate to the server, and MUST perform a private key-based encryption, which proves it has the private key associated with the certificate. Main character energy for real 🔑

As deployments need to protect sensitive data in transit, the client and server MUST negotiate a ciphersuite that 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 the server can use to perform these checks. 🧐

After the TLS negotiation finishes successfully, 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. Not safe, not it 🙅‍♀️

As LDAP includes native anonymous and plaintext authentication methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not used with LDAP. If the client requests an authorization identity in a form other than a DN, 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 ask the LDAP server to make use of security credentials exchanged by a lower layer. If a TLS session has not been set up between the client and server before the SASL EXTERNAL Bind request is made, and there is no other external source of authentication credentials (e.g. IP-level security [8]), or if the server did not request the client's authentication credentials while the TLS session was being set up, 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. Oof, that's a hard reset 😭

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 in the authzId form described below. 🌈

Other mechanisms define where the authorization identity goes in the credentials field. 🍒

The authorization identity is a string in the UTF-8 character set, matching 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 that support storing authentication credentials, such as passwords or certificates, in the directory MUST support the dnAuthzId choice. Non-negotiable, every server gets it 👑

The uAuthzId choice exists for client applications that want to authenticate to a local directory but don't know their own Distinguished Name or don't have a directory entry. The string is defined as only a sequence of UTF-8 encoded ISO 10646 characters, and any 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. Don't assume it's unique, babe 🙈

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:

TLS_NULL_WITH_NULL_NULL
TLS_RSA_WITH_NULL_MD5
TLS_RSA_WITH_NULL_SHA

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:

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. This one's the baseline, no exceptions 🫶

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. Iconic naming, no notes 💅

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. Okay, the drama is real and it's giving "encrypt everything" 🚨

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 by returning the unwillingToPerform result code rather than performing computationally expensive operations requested by unauthorized clients. 🛡️

A connection where 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 that can view and modify information in transit. Lowkey terrifying, so don't skip this step 😱

Additional security considerations relating to the EXTERNAL mechanism for negotiating 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 are greatly appreciated. The squad really showed up 🌟

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
USA

EMail: M.Wahl@innosoft.com
Harald Tveit Alvestrand
EDB Maxware
Pirsenteret
N-7462 Trondheim, Norway

Phone: +47 73 54 57 97
EMail: Harald@Alvestrand.no
Jeff Hodges
Oblix, Inc.
18922 Forge Drive
Cupertino, CA 95014
USA

Phone: +1-408-861-6656
EMail: JHodges@oblix.com
RL "Bob" Morgan
Computing and Communications
University of Washington
Seattle, WA 98105
USA

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

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