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diff --git a/imap/docs/rfc/rfc4422.txt b/imap/docs/rfc/rfc4422.txt new file mode 100644 index 00000000..049fa8c5 --- /dev/null +++ b/imap/docs/rfc/rfc4422.txt @@ -0,0 +1,1851 @@ + + + + + + +Network Working Group A. Melnikov, Ed. +Request for Comments: 4422 Isode Limited +Obsoletes: 2222 K. Zeilenga, Ed. +Category: Standards Track OpenLDAP Foundation + June 2006 + + + Simple Authentication and Security Layer (SASL) + +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 + + The Simple Authentication and Security Layer (SASL) is a framework + for providing authentication and data security services in + connection-oriented protocols via replaceable mechanisms. It + provides a structured interface between protocols and mechanisms. + The resulting framework allows new protocols to reuse existing + mechanisms and allows old protocols to make use of new mechanisms. + The framework also provides a protocol for securing subsequent + protocol exchanges within a data security layer. + + This document describes how a SASL mechanism is structured, describes + how protocols include support for SASL, and defines the protocol for + carrying a data security layer over a connection. In addition, this + document defines one SASL mechanism, the EXTERNAL mechanism. + + This document obsoletes RFC 2222. + + + + + + + + + + + + + +Melnikov & Zeilenga Standards Track [Page 1] + +RFC 4422 SASL June 2006 + + +Table of Contents + + 1. Introduction ....................................................3 + 1.1. Document Audiences .........................................4 + 1.2. Relationship to Other Documents ............................4 + 1.3. Conventions ................................................5 + 2. Identity Concepts ...............................................5 + 3. The Authentication Exchange .....................................6 + 3.1. Mechanism Naming ...........................................8 + 3.2. Mechanism Negotiation ......................................9 + 3.3. Request Authentication Exchange ............................9 + 3.4. Challenges and Responses ...................................9 + 3.4.1. Authorization Identity String ......................10 + 3.5. Aborting Authentication Exchanges .........................10 + 3.6. Authentication Outcome ....................................11 + 3.7. Security Layers ...........................................12 + 3.8. Multiple Authentications ..................................12 + 4. Protocol Requirements ..........................................13 + 5. Mechanism Requirements .........................................16 + 6. Security Considerations ........................................18 + 6.1. Active Attacks ............................................19 + 6.1.1. Hijack Attacks .....................................19 + 6.1.2. Downgrade Attacks ..................................19 + 6.1.3. Replay Attacks .....................................20 + 6.1.4. Truncation Attacks .................................20 + 6.1.5. Other Active Attacks ...............................20 + 6.2. Passive Attacks ...........................................20 + 6.3. Re-keying .................................................21 + 6.4. Other Considerations ......................................21 + 7. IANA Considerations ............................................22 + 7.1. SASL Mechanism Registry ...................................22 + 7.2. Registration Changes ......................................26 + 8. References .....................................................26 + 8.1. Normative References ......................................26 + 8.2. Informative References ....................................27 + 9. Acknowledgements ...............................................28 + Appendix A. The SASL EXTERNAL Mechanism ..........................29 + A.1. EXTERNAL Technical Specification ..........................29 + A.2. SASL EXTERNAL Examples ....................................30 + A.3. Security Considerations ...................................31 + Appendix B. Changes since RFC 2222 ...............................31 + + + + + + + + + + +Melnikov & Zeilenga Standards Track [Page 2] + +RFC 4422 SASL June 2006 + + +1. Introduction + + The Simple Authentication and Security Layer (SASL) is a framework + for providing authentication and data security services in + connection-oriented protocols via replaceable mechanisms. SASL + provides a structured interface between protocols and mechanisms. + SASL also provides a protocol for securing subsequent protocol + exchanges within a data security layer. The data security layer can + provide data integrity, data confidentiality, and other services. + + SASL's design is intended to allow new protocols to reuse existing + mechanisms without requiring redesign of the mechanisms and allows + existing protocols to make use of new mechanisms without redesign of + protocols. + + SASL is conceptually a framework that provides an abstraction layer + between protocols and mechanisms as illustrated in the following + diagram. + + SMTP LDAP XMPP Other protocols ... + \ | | / + \ | | / + SASL abstraction layer + / | | \ + / | | \ + EXTERNAL GSSAPI PLAIN Other mechanisms ... + + It is through the interfaces of this abstraction layer that the + framework allows any protocol to utilize any mechanism. While this + layer does generally hide the particulars of protocols from + mechanisms and the particulars of mechanisms from protocols, this + layer does not generally hide the particulars of mechanisms from + protocol implementations. For example, different mechanisms require + different information to operate, some of them use password-based + authentication, some of then require realm information, others make + use of Kerberos tickets, certificates, etc. Also, in order to + perform authorization, server implementations generally have to + implement identity mapping between authentication identities, whose + form is mechanism specific, and authorization identities, whose form + is application protocol specific. Section 2 discusses identity + concepts. + + It is possible to design and implement this framework in ways that do + abstract away particulars of similar mechanisms. Such a framework + implementation, as well as mechanisms implementations, could be + designed not only to be shared by multiple implementations of a + particular protocol but to be shared by implementations of multiple + protocols. + + + +Melnikov & Zeilenga Standards Track [Page 3] + +RFC 4422 SASL June 2006 + + + The framework incorporates interfaces with both protocols and + mechanisms in which authentication exchanges are carried out. + Section 3 discusses SASL authentication exchanges. + + To use SASL, each protocol (amongst other items) provides a method + for identifying which mechanism is to be used, a method for exchange + of mechanism-specific server-challenges and client-responses, and a + method for communicating the outcome of the authentication exchange. + Section 4 discusses SASL protocol requirements. + + Each SASL mechanism defines (amongst other items) a series of + server-challenges and client-responses that provide authentication + services and negotiate data security services. Section 5 discusses + SASL mechanism requirements. + + Section 6 discusses security considerations. Section 7 discusses + IANA considerations. Appendix A defines the SASL EXTERNAL mechanism. + +1.1. Document Audiences + + This document is written to serve several different audiences: + + - protocol designers using this specification to support + authentication in their protocol, + + - mechanism designers that define new SASL mechanisms, and + + - implementors of clients or servers for those protocols that + support SASL. + + While the document organization is intended to allow readers to focus + on details relevant to their engineering, readers are encouraged to + read and understand all aspects of this document. + +1.2. Relationship to Other Documents + + This document obsoletes RFC 2222. It replaces all portions of RFC + 2222 excepting sections 7.1 (the KERBEROS_IV mechanism), 7.2 (the + GSSAPI mechanism), 7.3 (the SKEY mechanism). The KERBEROS_IV and + SKEY mechanisms are now viewed as obsolete and their specifications + provided in RFC 2222 are Historic. The GSSAPI mechanism is now + separately specified [SASL-GSSAPI]. + + Appendix B provides a summary of changes since RFC 2222. + + + + + + + +Melnikov & Zeilenga Standards Track [Page 4] + +RFC 4422 SASL June 2006 + + +1.3. Conventions + + 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 BCP 14 [RFC2119]. + + Character names in this document use the notation for code points and + names from the Unicode Standard [Unicode]. For example, the letter + "a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>. + + Note: a glossary of terms used in Unicode can be found in [Glossary]. + Information on the Unicode character encoding model can be found in + [CharModel]. + + In examples, "C:" and "S:" indicate lines of data to be sent by the + client and server, respectively. Lines have been wrapped for + improved readability. + +2. Identity Concepts + + In practice, authentication and authorization may involve multiple + identities, possibly in different forms (simple username, Kerberos + principal, X.500 Distinguished Name, etc.), possibly with different + representations (e.g., ABNF-described UTF-8 encoded Unicode character + string, BER-encoded Distinguished Name). While technical + specifications often prescribe both the identity form and + representation used on the network, different identity forms and/or + representations may be (and often are) used within implementations. + How identities of different forms relate to each other is, generally, + a local matter. In addition, the forms and representations used + within an implementation are a local matter. + + However, conceptually, the SASL framework involves two identities: + + 1) an identity associated with the authentication credentials + (termed the authentication identity), and + + 2) an identity to act as (termed the authorization identity). + + SASL mechanism specifications describe the credential form(s) (e.g., + X.509 certificates, Kerberos tickets, simple username/password) used + to authenticate the client, including (where appropriate) the syntax + and semantics of authentication identities carried in the + credentials. SASL protocol specifications describe the identity + form(s) used in authorization and, in particular, prescribe the + syntax and semantics of the authorization identity character string + to be transferred by mechanisms. + + + + +Melnikov & Zeilenga Standards Track [Page 5] + +RFC 4422 SASL June 2006 + + + The client provides its credentials (which include or imply an + authentication identity) and, optionally, a character string + representing the requested authorization identity as part of the SASL + exchange. When this character string is omitted or empty, the client + is requesting to act as the identity associated with the credentials + (e.g., the user is requesting to act as the authentication identity). + + The server is responsible for verifying the client's credentials and + verifying that the identity it associates with the client's + credentials (e.g., the authentication identity) is allowed to act as + the authorization identity. A SASL exchange fails if either (or + both) of these verifications fails. (The SASL exchange may fail for + other reasons, such as service authorization failure.) + + However, the precise form(s) of the authentication identities (used + within the server in its verifications, or otherwise) and the precise + form(s) of the authorization identities (used in making authorization + decisions, or otherwise) are beyond the scope of SASL and this + specification. In some circumstances, the precise identity forms + used in some context outside of the SASL exchange may be dictated by + other specifications. For instance, an identity assumption + authorization (proxy authorization) policy specification may dictate + how authentication and authorization identities are represented in + policy statements. + +3. The Authentication Exchange + + Each authentication exchange consists of a message from the client to + the server requesting authentication via a particular mechanism, + followed by one or more pairs of challenges from the server and + responses from the client, followed by a message from the server + indicating the outcome of the authentication exchange. (Note: + exchanges may also be aborted as discussed in Section 3.5.) + + The following illustration provides a high-level overview of an + authentication exchange. + + C: Request authentication exchange + S: Initial challenge + C: Initial response + <additional challenge/response messages> + S: Outcome of authentication exchange + + If the outcome is successful and a security layer was negotiated, + this layer is then installed (see Section 3.7). This also applies to + the following illustrations. + + + + + +Melnikov & Zeilenga Standards Track [Page 6] + +RFC 4422 SASL June 2006 + + + Some mechanisms specify that the first data sent in the + authentication exchange is from the client to the server. Protocols + may provide an optional initial response field in the request message + to carry this data. Where the mechanism specifies that the first + data sent in the exchange is from the client to the server, the + protocol provides an optional initial response field, and the client + uses this field, the exchange is shortened by one round-trip: + + C: Request authentication exchange + Initial response + <additional challenge/response messages> + S: Outcome of authentication exchange + + Where the mechanism specifies that the first data sent in the + exchange is from the client to the server and this field is + unavailable or unused, the client request is followed by an empty + challenge. + + C: Request authentication exchange + S: Empty Challenge + C: Initial Response + <additional challenge/response messages> + S: Outcome of authentication exchange + + Should a client include an initial response in its request where the + mechanism does not allow the client to send data first, the + authentication exchange fails. + + Some mechanisms specify that the server is to send additional data to + the client when indicating a successful outcome. Protocols may + provide an optional additional data field in the outcome message to + carry this data. Where the mechanism specifies that the server is to + return additional data with the successful outcome, the protocol + provides an optional additional data field in the outcome message, + and the server uses this field, the exchange is shortened by one + round-trip: + + C: Request authentication exchange + S: Initial challenge + C: Initial response + <additional challenge/response messages> + S: Outcome of authentication exchange with + additional data with success + + Where the mechanism specifies that the server is to return additional + data to the client with a successful outcome and this field is + unavailable or unused, the additional data is sent as a challenge + whose response is empty. After receiving this response, the server + then indicates the successful outcome. + + + +Melnikov & Zeilenga Standards Track [Page 7] + +RFC 4422 SASL June 2006 + + + C: Request authentication exchange + S: Initial challenge + C: Initial response + <additional challenge/response messages> + S: Additional data challenge + C: Empty Response + S: Outcome of authentication exchange + + Where mechanisms specify that the first data sent in the exchange is + from the client to the server and additional data is sent to the + client along with indicating a successful outcome, and the protocol + provides fields supporting both, then the exchange takes two fewer + round-trips: + + C: Request authentication exchange + Initial response + <additional challenge/response messages> + S: Outcome of authentication exchange + with additional data with success + + instead of: + + C: Request authentication exchange + S: Empty Challenge + C: Initial Response + <additional challenge/response messages> + S: Additional data challenge + C: Empty Response + S: Outcome of authentication exchange + +3.1. Mechanism Naming + + SASL mechanisms are named by character strings, from 1 to 20 + characters in length, consisting of ASCII [ASCII] uppercase letters, + digits, hyphens, and/or underscores. In the following Augmented + Backus-Naur Form (ABNF) [RFC4234] grammar, the <sasl-mech> production + defines the syntax of a SASL mechanism name. + + sasl-mech = 1*20mech-char + mech-char = UPPER-ALPHA / DIGIT / HYPHEN / UNDERSCORE + ; mech-char is restricted to A-Z (uppercase only), 0-9, -, and _ + ; from ASCII character set. + + UPPER-ALPHA = %x41-5A ; A-Z (uppercase only) + DIGIT = %x30-39 ; 0-9 + HYPHEN = %x2D ; hyphen (-) + UNDERSCORE = %x5F ; underscore (_) + + SASL mechanism names are registered as discussed in Section 7.1. + + + +Melnikov & Zeilenga Standards Track [Page 8] + +RFC 4422 SASL June 2006 + + +3.2. Mechanism Negotiation + + Mechanism negotiation is protocol specific. + + Commonly, a protocol will specify that the server advertises + supported and available mechanisms to the client via some facility + provided by the protocol, and the client will then select the "best" + mechanism from this list that it supports and finds suitable. + + Note that the mechanism negotiation is not protected by the + subsequent authentication exchange and hence is subject to downgrade + attacks if not protected by other means. + + To detect downgrade attacks, a protocol can allow the client to + discover available mechanisms subsequent to the authentication + exchange and installation of data security layers with at least data + integrity protection. This allows the client to detect changes to + the list of mechanisms supported by the server. + +3.3. Request Authentication Exchange + + The authentication exchange is initiated by the client by requesting + authentication via a mechanism it specifies. The client sends a + message that contains the name of the mechanism to the server. The + particulars of the message are protocol specific. + + Note that the name of the mechanism is not protected by the + mechanism, and hence is subject to alteration by an attacker if not + integrity protected by other means. + + Where the mechanism is defined to allow the client to send data + first, and the protocol's request message includes an optional + initial response field, the client may include the response to the + initial challenge in the authentication request message. + +3.4. Challenges and Responses + + The authentication exchange involves one or more pairs of server- + challenges and client-responses, the particulars of which are + mechanism specific. These challenges and responses are enclosed in + protocol messages, the particulars of which are protocol specific. + + Through these challenges and responses, the mechanism may: + + - authenticate the client to the server, + + - authenticate the server to the client, + + + + +Melnikov & Zeilenga Standards Track [Page 9] + +RFC 4422 SASL June 2006 + + + - transfer an authorization identity string, + + - negotiate a security layer, and + + - provide other services. + + The negotiation of the security layer may involve negotiation of the + security services to be provided in the layer, how these services + will be provided, and negotiation of a maximum cipher-text buffer + size each side is able to receive in the layer (see Section 3.6). + + After receiving an authentication request or any client response, the + server may issue a challenge, abort the exchange, or indicate the + outcome of an exchange. After receiving a challenge, a client + mechanism may issue a response or abort the exchange. + +3.4.1. Authorization Identity String + + The authorization identity string is a sequence of zero or more + Unicode [Unicode] characters, excluding the NUL (U+0000) character, + representing the identity to act as. + + If the authorization identity string is absent, the client is + requesting to act as the identity the server associates with the + client's credentials. An empty string is equivalent to an absent + authorization identity. + + A non-empty authorization identity string indicates that the client + wishes to act as the identity represented by the string. In this + case, the form of identity represented by the string, as well as the + precise syntax and semantics of the string, is protocol specific. + + While the character encoding schema used to transfer the + authorization identity string in the authentication exchange is + mechanism specific, mechanisms are expected to be capable of carrying + the entire Unicode repertoire (with the exception of the NUL + character). + +3.5. Aborting Authentication Exchanges + + A client or server may desire to abort an authentication exchange if + it is unwilling or unable to continue (or enter into). + + A client may abort the authentication exchange by sending a message, + the particulars of which are protocol specific, to the server, + indicating that the exchange is aborted. The server may be required + by the protocol to return a message in response to the client's abort + message. + + + +Melnikov & Zeilenga Standards Track [Page 10] + +RFC 4422 SASL June 2006 + + + Likewise, a server may abort the authentication exchange by sending a + message, the particulars of which are protocol specific, to the + client, indicating that the exchange is aborted. + +3.6. Authentication Outcome + + At the conclusion of the authentication exchange, the server sends a + message, the particulars of which are protocol specific, to the + client indicating the outcome of the exchange. + + The outcome is not successful if + + - the authentication exchange failed for any reason, + + - the client's credentials could not be verified, + + - the server cannot associate an identity with the client's + credentials, + + - the client-provided authorization identity string is malformed, + + - the identity associated with the client's credentials is not + authorized to act as the requested authorization identity, + + - the negotiated security layer (or lack thereof) is not + suitable, or + + - the server is not willing to provide service to the client for + any reason. + + The protocol may include an optional additional data field in this + outcome message. This field can only include additional data when + the outcome is successful. + + If the outcome is successful and a security layer was negotiated, + this layer is then installed. If the outcome is unsuccessful, or a + security layer was not negotiated, any existing security is left in + place. + + The outcome message provided by the server can provide a way for the + client to distinguish between errors that are best dealt with by re- + prompting the user for her credentials, errors that are best dealt + with by telling the user to try again later, and errors where the + user must contact a system administrator for resolution (see the SYS + and AUTH POP Response Codes [RFC3206] specification for an example). + This distinction is particularly useful during scheduled server + maintenance periods as it reduces support costs. It is also + important that the server can be configured such that the outcome + + + +Melnikov & Zeilenga Standards Track [Page 11] + +RFC 4422 SASL June 2006 + + + message will not distinguish between a valid user with invalid + credentials and an invalid user. + +3.7. Security Layers + + SASL mechanisms may offer a wide range of services in security + layers. Typical services include data integrity and data + confidentiality. SASL mechanisms that do not provide a security + layer are treated as negotiating no security layer. + + If use of a security layer is negotiated in the authentication + protocol exchange, the layer is installed by the server after + indicating the outcome of the authentication exchange and installed + by the client upon receipt of the outcome indication. In both cases, + the layer is installed before transfer of further protocol data. The + precise position upon which the layer takes effect in the protocol + data stream is protocol specific. + + Once the security layer is in effect in the protocol data stream, it + remains in effect until either a subsequently negotiated security + layer is installed or the underlying transport connection is closed. + + When in effect, the security layer processes protocol data into + buffers of protected data. If at any time the security layer is + unable or unwilling to continue producing buffers protecting protocol + data, the underlying transport connection MUST be closed. If the + security layer is not able to decode a received buffer, the + underlying connection MUST be closed. In both cases, the underlying + transport connection SHOULD be closed gracefully. + + Each buffer of protected data is transferred over the underlying + transport connection as a sequence of octets prepended with a four- + octet field in network byte order that represents the length of the + buffer. The length of the protected data buffer MUST be no larger + than the maximum size that the other side expects. Upon the receipt + of a length field whose value is greater than the maximum size, the + receiver SHOULD close the connection, as this might be a sign of an + attack. + + The maximum size that each side expects is fixed by the mechanism, + either through negotiation or by its specification. + +3.8. Multiple Authentications + + Unless explicitly permitted in the protocol (as stated in the + protocol's technical specification), only one successful SASL + authentication exchange may occur in a protocol session. In this + + + + +Melnikov & Zeilenga Standards Track [Page 12] + +RFC 4422 SASL June 2006 + + + case, once an authentication exchange has successfully completed, + further attempts to initiate an authentication exchange fail. + + Where multiple successful SASL authentication exchanges are permitted + in the protocol, then in no case may multiple SASL security layers be + simultaneously in effect. If a security layer is in effect and a + subsequent SASL negotiation selects a second security layer, then the + second security layer replaces the first. If a security layer is in + effect and a subsequent SASL negotiation selects no security layer, + the original security layer remains in effect. + + Where multiple successful SASL negotiations are permitted in the + protocol, the effect of a failed SASL authentication exchange upon + the previously established authentication and authorization state is + protocol specific. The protocol's technical specification should be + consulted to determine whether the previous authentication and + authorization state remains in force, or changed to an anonymous + state, or otherwise was affected. Regardless of the protocol- + specific effect upon previously established authentication and + authorization state, the previously negotiated security layer remains + in effect. + +4. Protocol Requirements + + In order for a protocol to offer SASL services, its specification + MUST supply the following information: + + 1) A service name, to be selected from registry of "service" elements + for the Generic Security Service Application Program Interface + (GSSAPI) host-based service name form, as described in Section 4.1 + of [RFC2743]. Note that this registry is shared by all GSSAPI and + SASL mechanisms. + + 2) Detail any mechanism negotiation facility that the protocol + provides (see Section 3.2). + + A protocol SHOULD specify a facility through which the client may + discover, both before initiation of the SASL exchange and after + installing security layers negotiated by the exchange, the names + of the SASL mechanisms that the server makes available to the + client. The latter is important to allow the client to detect + downgrade attacks. This facility is typically provided through + the protocol's extensions or capabilities discovery facility. + + 3) Definition of the messages necessary for authentication exchange, + including the following: + + + + + +Melnikov & Zeilenga Standards Track [Page 13] + +RFC 4422 SASL June 2006 + + + a) A message to initiate the authentication exchange (see Section + 3.3). + + This message MUST contain a field for carrying the name of the + mechanism selected by the client. + + This message SHOULD contain an optional field for carrying an + initial response. If the message is defined with this field, + the specification MUST describe how messages with an empty + initial response are distinguished from messages with no + initial response. This field MUST be capable of carrying + arbitrary sequences of octets (including zero-length sequences + and sequences containing zero-valued octets). + + b) Messages to transfer server challenges and client responses + (see Section 3.4). + + Each of these messages MUST be capable of carrying arbitrary + sequences of octets (including zero-length sequences and + sequences containing zero-valued octets). + + c) A message to indicate the outcome of the authentication + exchange (see Section 3.6). + + This message SHOULD contain an optional field for carrying + additional data with a successful outcome. If the message is + defined with this field, the specification MUST describe how + messages with an empty additional data are distinguished from + messages with no additional data. This field MUST be capable + of carrying arbitrary sequences of octets (including zero- + length sequences and sequences containing zero-valued octets). + + 4) Prescribe the syntax and semantics of non-empty authorization + identity strings (see Section 3.4.1). + + In order to avoid interoperability problems due to differing + normalizations, the protocol specification MUST detail precisely + how and where (client or server) non-empty authorization identity + strings are prepared, including all normalizations, for comparison + and other applicable functions to ensure proper function. + + Specifications are encouraged to prescribe use of existing + authorization identity forms as well as existing string + representations, such as simple user names [RFC4013]. + + Where the specification does not precisely prescribe how + identities in SASL relate to identities used elsewhere in the + protocol, for instance, in access control policy statements, it + + + +Melnikov & Zeilenga Standards Track [Page 14] + +RFC 4422 SASL June 2006 + + + may be appropriate for the protocol to provide a facility by which + the client can discover information (such as the representation of + the identity used in making access control decisions) about + established identities for these uses. + + 5) Detail any facility the protocol provides that allows the client + and/or server to abort authentication exchange (see Section 3.5). + + Protocols that support multiple authentications typically allow a + client to abort an ongoing authentication exchange by initiating a + new authentication exchange. Protocols that do not support + multiple authentications may require the client to close the + connection and start over to abort an ongoing authentication + exchange. + + Protocols typically allow the server to abort ongoing + authentication exchanges by returning a non-successful outcome + message. + + 6) Identify precisely where newly negotiated security layers start to + take effect, in both directions (see Section 3.7). + + Typically, specifications require security layers to start taking + effect on the first octet following the outcome message in data + being sent by the server and on the first octet sent after receipt + of the outcome message in data being sent by the client. + + 7) If the protocol supports other layered security services, such as + Transport Layer Security (TLS) [RFC4346], the specification MUST + prescribe the order in which security layers are applied to + protocol data. + + For instance, where a protocol supports both TLS and SASL security + layers, the specification could prescribe any of the following: + + a) SASL security layer is always applied first to data being sent + and, hence, applied last to received data, + + b) SASL security layer is always applied last to data being sent + and, hence, applied first to received data, + + c) Layers are applied in the order in which they were installed, + + d) Layers are applied in the reverse order in which they were + installed, or + + e) Both TLS and SASL security layers cannot be installed. + + + + +Melnikov & Zeilenga Standards Track [Page 15] + +RFC 4422 SASL June 2006 + + + 8) Indicate whether the protocol supports multiple authentications + (see Section 3.8). If so, the protocol MUST detail the effect a + failed SASL authentication exchange will have upon a previously + established authentication and authorization state. + + Protocol specifications SHOULD avoid stating implementation + requirements that would hinder replacement of applicable mechanisms. + In general, protocol specifications SHOULD be mechanism neutral. + There are a number of reasonable exceptions to this recommendation, + including + + - detailing how credentials (which are mechanism specific) are + managed in the protocol, + + - detailing how authentication identities (which are mechanism + specific) and authorization identities (which are protocol + specific) relate to each other, and + + - detailing which mechanisms are applicable to the protocol. + +5. Mechanism Requirements + + SASL mechanism specifications MUST supply the following information: + + 1) The name of the mechanism (see Section 3.1). This name MUST be + registered as discussed in Section 7.1. + + 2) A definition of the server-challenges and client-responses of the + authentication exchange, as well as the following: + + a) An indication of whether the mechanism is client-first, + variable, or server-first. If a SASL mechanism is defined as + client-first and the client does not send an initial response + in the authentication request, then the first server challenge + MUST be empty (the EXTERNAL mechanism is an example of this + case). If a SASL mechanism is defined as variable, then the + specification needs to state how the server behaves when the + initial client response in the authentication request is + omitted (the DIGEST-MD5 mechanism [DIGEST-MD5] is an example of + this case). If a SASL mechanism is defined as server-first, + then the client MUST NOT send an initial client response in the + authentication request (the CRAM-MD5 mechanism [CRAM-MD5] is an + example of this case). + + b) An indication of whether the server is expected to provide + additional data when indicating a successful outcome. If so, + if the server sends the additional data as a challenge, the + + + + +Melnikov & Zeilenga Standards Track [Page 16] + +RFC 4422 SASL June 2006 + + + specification MUST indicate that the response to this challenge + is an empty response. + + SASL mechanisms SHOULD be designed to minimize the number of + challenges and responses necessary to complete the exchange. + + 3) An indication of whether the mechanism is capable of transferring + authorization identity strings (see Section 3.4.1). While some + legacy mechanisms are incapable of transmitting an authorization + identity (which means that for these mechanisms, the authorization + identity is always the empty string), newly defined mechanisms + SHOULD be capable of transferring authorization identity strings. + The mechanism SHOULD NOT be capable of transferring both no + authorization identity string and an empty authorization identity. + + Mechanisms that are capable of transferring an authorization + identity string MUST be capable of transferring arbitrary non- + empty sequences of Unicode characters, excluding those that + contain the NUL (U+0000) character. Mechanisms SHOULD use the + UTF-8 [RFC3629] transformation format. The specification MUST + detail how any Unicode code points special to the mechanism that + might appear in the authorization identity string are escaped to + avoid ambiguity during decoding of the authorization identity + string. Typically, mechanisms that have special characters + require these special characters to be escaped or encoded in the + character string (after encoding it in a particular Unicode + transformation format) using a data encoding scheme such as Base64 + [RFC3548]. + + 4) The specification MUST detail whether the mechanism offers a + security layer. If the mechanism does, the specification MUST + detail the security and other services offered in the layer as + well as how these services are to be implemented. + + 5) If the underlying cryptographic technology used by a mechanism + supports data integrity, then the mechanism specification MUST + integrity protect the transmission of an authorization identity + and the negotiation of the security layer. + + SASL mechanisms SHOULD be protocol neutral. + + SASL mechanisms SHOULD reuse existing credential and identity forms, + as well as associated syntaxes and semantics. + + SASL mechanisms SHOULD use the UTF-8 transformation format [RFC3629] + for encoding Unicode [Unicode] code points for transfer. + + + + + +Melnikov & Zeilenga Standards Track [Page 17] + +RFC 4422 SASL June 2006 + + + In order to avoid interoperability problems due to differing + normalizations, when a mechanism calls for character data (other than + the authorization identity string) to be used as input to a + cryptographic and/or comparison function, the specification MUST + detail precisely how and where (client or server) the character data + is to be prepared, including all normalizations, for input into the + function to ensure proper operation. + + For simple user names and/or passwords in authentication credentials, + SASLprep [RFC4013] (a profile of the StringPrep [RFC3454] preparation + algorithm), SHOULD be specified as the preparation algorithm. + + The mechanism SHOULD NOT use the authorization identity string in + generation of any long-term cryptographic keys or hashes as there is + no requirement that the authorization identity string be canonical. + Long-term, here, means a term longer than the duration of the + authentication exchange in which they were generated. That is, as + different clients (of the same or different protocol) may provide + different authorization identity strings that are semantically + equivalent, use of authorization identity strings in generation of + cryptographic keys and hashes will likely lead to interoperability + and other problems. + +6. Security Considerations + + Security issues are discussed throughout this memo. + + Many existing SASL mechanisms do not provide adequate protection + against passive attacks, let alone active attacks, in the + authentication exchange. Many existing SASL mechanisms do not offer + security layers. It is hoped that future SASL mechanisms will + provide strong protection against passive and active attacks in the + authentication exchange, as well as security layers with strong basic + data security features (e.g., data integrity and data + confidentiality) services. It is also hoped that future mechanisms + will provide more advanced data security services like re-keying (see + Section 6.3). + + Regardless, the SASL framework is susceptible to downgrade attacks. + Section 6.1.2 offers a variety of approaches for preventing or + detecting these attacks. In some cases, it is appropriate to use + data integrity protective services external to SASL (e.g., TLS) to + protect against downgrade attacks in SASL. Use of external + protective security services is also important when the mechanisms + available do not themselves offer adequate integrity and/or + confidentiality protection of the authentication exchange and/or + protocol data. + + + + +Melnikov & Zeilenga Standards Track [Page 18] + +RFC 4422 SASL June 2006 + + +6.1. Active Attacks + +6.1.1. Hijack Attacks + + When the client selects a SASL security layer with at least integrity + protection, this protection serves as a counter-measure against an + active attacker hijacking the connection and modifying protocol data + sent after establishment of the security layer. Implementations + SHOULD close the connection when the security services in a SASL + security layer report protocol data report lack of data integrity. + +6.1.2. Downgrade Attacks + + It is important that any security-sensitive protocol negotiations be + performed after installation of a security layer with data integrity + protection. Protocols should be designed such that negotiations + performed prior to this installation should be revalidated after + installation is complete. Negotiation of the SASL mechanism is + security sensitive. + + When a client negotiates the authentication mechanism with the server + and/or other security features, it is possible for an active attacker + to cause a party to use the least secure security services available. + For instance, an attacker can modify the server-advertised mechanism + list or can modify the client-advertised security feature list within + a mechanism response. To protect against this sort of attack, + implementations SHOULD NOT advertise mechanisms and/or features that + cannot meet their minimum security requirements, SHOULD NOT enter + into or continue authentication exchanges that cannot meet their + minimum security requirements, and SHOULD verify that completed + authentication exchanges result in security services that meet their + minimum security requirements. Note that each endpoint needs to + independently verify that its security requirements are met. + + In order to detect downgrade attacks to the least (or less) secure + mechanism supported, the client can discover the SASL mechanisms that + the server makes available both before the SASL authentication + exchange and after the negotiated SASL security layer (with at least + data integrity protection) has been installed through the protocol's + mechanism discovery facility. If the client finds that the + integrity-protected list (the list obtained after the security layer + was installed) contains a stronger mechanism than those in the + previously obtained list, the client should assume that the + previously obtained list was modified by an attacker and SHOULD close + the underlying transport connection. + + The client's initiation of the SASL exchange, including the selection + of a SASL mechanism, is done in the clear and may be modified by an + + + +Melnikov & Zeilenga Standards Track [Page 19] + +RFC 4422 SASL June 2006 + + + active attacker. It is important for any new SASL mechanisms to be + designed such that an active attacker cannot obtain an authentication + with weaker security properties by modifying the SASL mechanism name + and/or the challenges and responses. + + Multi-level negotiation of security features is prone to downgrade + attack. Protocol designers should avoid offering higher-level + negotiation of security features in protocols (e.g., above SASL + mechanism negotiation) and mechanism designers should avoid lower- + level negotiation of security features in mechanisms (e.g., below + SASL mechanism negotiation). + +6.1.3. Replay Attacks + + Some mechanisms may be subject to replay attacks unless protected by + external data security services (e.g., TLS). + +6.1.4. Truncation Attacks + + Most existing SASL security layers do not themselves offer protection + against truncation attack. In a truncation attack, the active + attacker causes the protocol session to be closed, causing a + truncation of the possibly integrity-protected data stream that leads + to behavior of one or both the protocol peers that inappropriately + benefits the attacker. Truncation attacks are fairly easy to defend + against in connection-oriented application-level protocols. A + protocol can defend against these attacks by ensuring that each + information exchange has a clear final result and that each protocol + session has a graceful closure mechanism, and that these are + integrity protected. + +6.1.5. Other Active Attacks + + When use of a security layer is negotiated by the authentication + protocol exchange, the receiver SHOULD handle gracefully any + protected data buffer larger than the defined/negotiated maximal + size. In particular, it MUST NOT blindly allocate the amount of + memory specified in the buffer size field, as this might cause the + "out of memory" condition. If the receiver detects a large block, it + SHOULD close the connection. + +6.2. Passive Attacks + + Many mechanisms are subject to various passive attacks, including + simple eavesdropping of unprotected credential information as well as + online and offline dictionary attacks of protected credential + information. + + + + +Melnikov & Zeilenga Standards Track [Page 20] + +RFC 4422 SASL June 2006 + + +6.3. Re-keying + + The secure or administratively permitted lifetimes of SASL + mechanisms' security layers are finite. Cryptographic keys weaken as + they are used and as time passes; the more time and/or cipher-text + that a cryptanalyst has after the first use of the a key, the easier + it is for the cryptanalyst to mount attacks on the key. + + Administrative limits on a security layer's lifetime may take the + form of time limits expressed in X.509 certificates, in Kerberos V + tickets, or in directories, and are often desired. In practice, one + likely effect of administrative lifetime limits is that applications + may find that security layers stop working in the middle of + application protocol operation, such as, perhaps, during large data + transfers. As the result of this, the connection will be closed (see + Section 3.7), which will result in an unpleasant user experience. + + Re-keying (key renegotiation process) is a way of addressing the + weakening of cryptographic keys. The SASL framework does not itself + provide for re-keying; SASL mechanisms may. Designers of future SASL + mechanisms should consider providing re-keying services. + + Implementations that wish to re-key SASL security layers where the + mechanism does not provide for re-keying SHOULD reauthenticate the + same IDs and replace the expired or soon-to-expire security layers. + This approach requires support for reauthentication in the + application protocols (see Section 3.8). + +6.4. Other Considerations + + Protocol designers and implementors should understand the security + considerations of mechanisms so they may select mechanisms that are + applicable to their needs. + + Distributed server implementations need to be careful in how they + trust other parties. In particular, authentication secrets should + only be disclosed to other parties that are trusted to manage and use + those secrets in a manner acceptable to the disclosing party. + Applications using SASL assume that SASL security layers providing + data confidentiality are secure even when an attacker chooses the + text to be protected by the security layer. Similarly, applications + assume that the SASL security layer is secure even if the attacker + can manipulate the cipher-text output of the security layer. New + SASL mechanisms are expected to meet these assumptions. + + + + + + + +Melnikov & Zeilenga Standards Track [Page 21] + +RFC 4422 SASL June 2006 + + + Unicode security considerations [UTR36] apply to authorization + identity strings, as well as UTF-8 [RFC3629] security considerations + where UTF-8 is used. SASLprep [RFC4013] and StringPrep [RFC3454] + security considerations also apply where used. + +7. IANA Considerations + +7.1. SASL Mechanism Registry + + The SASL mechanism registry is maintained by IANA. The registry is + currently available at <http://www.iana.org/assignments/sasl- + mechanisms>. + + The purpose of this registry is not only to ensure uniqueness of + values used to name SASL mechanisms, but also to provide a definitive + reference to technical specifications detailing each SASL mechanism + available for use on the Internet. + + There is no naming convention for SASL mechanisms; any name that + conforms to the syntax of a SASL mechanism name can be registered. + + The procedure detailed in Section 7.1.1 is to be used for + registration of a value naming a specific individual mechanism. + + The procedure detailed in Section 7.1.2 is to be used for + registration of a value naming a family of related mechanisms. + + Comments may be included in the registry as discussed in Section + 7.1.3 and may be changed as discussed in Section 7.1.4. + + The SASL mechanism registry has been updated to reflect that this + document provides the definitive technical specification for SASL and + that this section provides the registration procedures for this + registry. + + + + + + + + + + + + + + + + + +Melnikov & Zeilenga Standards Track [Page 22] + +RFC 4422 SASL June 2006 + + +7.1.1. Mechanism Name Registration Procedure + + IANA will register new SASL mechanism names on a First Come First + Served basis, as defined in BCP 26 [RFC2434]. IANA has the right to + reject obviously bogus registration requests, but will perform no + review of claims made in the registration form. + + Registration of a SASL mechanism is requested by filling in the + following template: + + Subject: Registration of SASL mechanism X + + SASL mechanism name (or prefix for the family): + + Security considerations: + + Published specification (recommended): + + Person & email address to contact for further information: + + Intended usage: (One of COMMON, LIMITED USE, or OBSOLETE) + + Owner/Change controller: + + Note: (Any other information that the author deems relevant may be + added here.) + + and sending it via electronic mail to IANA at <iana@iana.org>. + + While this registration procedure does not require expert review, + authors of SASL mechanisms are encouraged to seek community review + and comment whenever that is feasible. Authors may seek community + review by posting a specification of their proposed mechanism as an + Internet-Draft. SASL mechanisms intended for widespread use should + be standardized through the normal IETF process, when appropriate. + + + + + + + + + + + + + + + + +Melnikov & Zeilenga Standards Track [Page 23] + +RFC 4422 SASL June 2006 + + +7.1.2. Family Name Registration Procedure + + As noted above, there is no general naming convention for SASL + mechanisms. However, specifications may reserve a portion of the + SASL mechanism namespace for a set of related SASL mechanisms, a + "family" of SASL mechanisms. Each family of SASL mechanisms is + identified by a unique prefix, such as X-. Registration of new SASL + mechanism family names requires expert review as defined in BCP 26 + [RFC2434]. + + Registration of a SASL family name is requested by filling in the + following template: + + Subject: Registration of SASL mechanism family X + + SASL family name (or prefix for the family): + + Security considerations: + + Published specification (recommended): + + Person & email address to contact for further information: + + Intended usage: (One of COMMON, LIMITED USE, or OBSOLETE) + + Owner/Change controller: + + Note: (Any other information that the author deems relevant may be + added here.) + + and sending it via electronic mail to the IETF SASL mailing list at + <ietf-sasl@imc.org> and carbon copying IANA at <iana@iana.org>. + After allowing two weeks for community input on the IETF SASL mailing + list, the expert will determine the appropriateness of the + registration request and either approve or disapprove the request + with notice to the requestor, the mailing list, and IANA. + + The review should focus on the appropriateness of the requested + family name for the proposed use and the appropriateness of the + proposed naming and registration plan for existing and future + mechanism names in the family. The scope of this request review may + entail consideration of relevant aspects of any provided technical + specification, such as their IANA Considerations section. However, + this review is narrowly focused on the appropriateness of the + requested registration and not on the overall soundness of any + provided technical specification. + + + + + +Melnikov & Zeilenga Standards Track [Page 24] + +RFC 4422 SASL June 2006 + + + Authors are encouraged to pursue community review by posting the + technical specification as an Internet-Draft and soliciting comment + by posting to appropriate IETF mailing lists. + +7.1.3. Comments on SASL Mechanism Registrations + + Comments on a registered SASL mechanism/family should first be sent + to the "owner" of the mechanism/family and/or to the <ietf- + sasl@imc.org> mailing list. + + Submitters of comments may, after a reasonable attempt to contact the + owner, request IANA to attach their comment to the SASL mechanism + registration itself by sending mail to <iana@iana.org>. At IANA's + sole discretion, IANA may attach the comment to the SASL mechanism's + registration. + +7.1.4. Change Control + + Once a SASL mechanism registration has been published by IANA, the + author may request a change to its definition. The change request + follows the same procedure as the registration request. + + The owner of a SASL mechanism may pass responsibility for the SASL + mechanism to another person or agency by informing IANA; this can be + done without discussion or review. + + The IESG may reassign responsibility for a SASL mechanism. The most + common case of this will be to enable changes to be made to + mechanisms where the author of the registration has died, has moved + out of contact, or is otherwise unable to make changes that are + important to the community. + + SASL mechanism registrations may not be deleted; mechanisms that are + no longer believed appropriate for use can be declared OBSOLETE by a + change to their "intended usage" field; such SASL mechanisms will be + clearly marked in the lists published by IANA. + + The IESG is considered to be the owner of all SASL mechanisms that + are on the IETF standards track. + + + + + + + + + + + + +Melnikov & Zeilenga Standards Track [Page 25] + +RFC 4422 SASL June 2006 + + +7.2. Registration Changes + + The IANA has updated the SASL mechanisms registry as follows: + + 1) Changed the "Intended usage" of the KERBEROS_V4 and SKEY mechanism + registrations to OBSOLETE. + + 2) Changed the "Published specification" of the EXTERNAL mechanism to + this document as indicated below: + + Subject: Updated Registration of SASL mechanism EXTERNAL + Family of SASL mechanisms: NO + SASL mechanism name: EXTERNAL + Security considerations: See A.3 of RFC 4422 + Published specification (optional, recommended): RFC 4422 + Person & email address to contact for further information: + Alexey Melnikov <Alexey.Melnikov@isode.com> + Intended usage: COMMON + Owner/Change controller: IESG <iesg@ietf.org> + Note: Updates existing entry for EXTERNAL + +8. References + +8.1. Normative References + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, March 1997. + + [RFC2244] Newman, C. and J. G. Myers, "ACAP -- Application + Configuration Access Protocol", RFC 2244, November + 1997. + + [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing + an IANA Considerations Section in RFCs", BCP 26, RFC + 2434, October 1998. + + [RFC2743] Linn, J., "Generic Security Service Application Program + Interface Version 2, Update 1", RFC 2743, January 2000. + + [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of + Internationalized Strings ("stringprep")", RFC 3454, + December 2002. + + [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO + 10646", STD 63, RFC 3629, November 2003. + + [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User + Names and Passwords", RFC 4013, February 2005. + + + +Melnikov & Zeilenga Standards Track [Page 26] + +RFC 4422 SASL June 2006 + + + [RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax + Specifications: ABNF", RFC 4234, October 2005. + + [ASCII] Coded Character Set--7-bit American Standard Code for + Information Interchange, ANSI X3.4-1986. + + [Unicode] The Unicode Consortium, "The Unicode Standard, Version + 3.2.0" is defined by "The Unicode Standard, Version + 3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201- + 61633-5), as amended by the "Unicode Standard Annex + #27: Unicode 3.1" + (http://www.unicode.org/reports/tr27/) and by the + "Unicode Standard Annex #28: Unicode 3.2" + (http://www.unicode.org/reports/tr28/). + + [CharModel] Whistler, K. and M. Davis, "Unicode Technical Report + #17, Character Encoding Model", UTR17, + <http://www.unicode.org/unicode/reports/tr17/>, August + 2000. + + [Glossary] The Unicode Consortium, "Unicode Glossary", + <http://www.unicode.org/glossary/>. + +8.2. Informative References + + [RFC3206] Gellens, R., "The SYS and AUTH POP Response Codes", RFC + 3206, February 2002. + + [RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data + Encodings", RFC 3548, July 2003. + + [RFC4301] Kent, S. and K. Seo, "Security Architecture for the + Internet Protocol", RFC 4301, December 2005. + + [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer + Security (TLS) Protocol Version 1.1", RFC 4346, April + 2006. + + [SASL-GSSAPI] Melnikov, A. (Editor), "The Kerberos V5 ("GSSAPI") SASL + Mechanism", Work in Progress, May 2006. + + [UTR36] Davis, M., "(Draft) Unicode Technical Report #36, + Character Encoding Model", UTR17, + <http://www.unicode.org/unicode/reports/tr36/>, + February 2005. + + [CRAM-MD5] Nerenberg, L., "The CRAM-MD5 SASL Mechanism", Work in + Progress. + + + +Melnikov & Zeilenga Standards Track [Page 27] + +RFC 4422 SASL June 2006 + + + [DIGEST-MD5] Leach, P., C. Newman, and A. Melnikov, "Using Digest + Authentication as a SASL Mechanism", Work in Progress, + March 2006. + +9. Acknowledgements + + This document is a revision of RFC 2222 written by John Myers. + + This revision is a product of the IETF Simple Authentication and + Security Layer (SASL) Working Group. + + The following individuals contributed significantly to this revision: + Abhijit Menon-Sen, Hallvard Furuseth, Jeffrey Hutzelman, John Myers, + Luke Howard, Magnus Nystrom, Nicolas Williams, Peter Saint-Andre, RL + 'Bob' Morgan, Rob Siemborski, Sam Hartman, Simon Josefsson, Tim + Alsop, and Tony Hansen. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Melnikov & Zeilenga Standards Track [Page 28] + +RFC 4422 SASL June 2006 + + +Appendix A. The SASL EXTERNAL Mechanism + + This appendix is normative. + + The EXTERNAL mechanism allows a client to request the server to use + credentials established by means external to the mechanism to + authenticate the client. The external means may be, for instance, IP + Security [RFC4301] or TLS [RFC4346] services. In absence of some a + priori agreement between the client and the server, the client cannot + make any assumption as to what external means the server has used to + obtain the client's credentials, nor make an assumption as to the + form of credentials. For example, the client cannot assume that the + server will use the credentials the client has established via TLS. + +A.1. EXTERNAL Technical Specification + + The name of this mechanism is "EXTERNAL". + + The mechanism does not provide a security layer. + + The mechanism is capable of transferring an authorization identity + string. If empty, the client is requesting to act as the identity + the server has associated with the client's credentials. If non- + empty, the client is requesting to act as the identity represented by + the string. + + The client is expected to send data first in the authentication + exchange. Where the client does not provide an initial response data + in its request to initiate the authentication exchange, the server is + to respond to the request with an empty initial challenge and then + the client is to provide its initial response. + + The client sends the initial response containing the UTF-8 [RFC3629] + encoding of the requested authorization identity string. This + response is non-empty when the client is requesting to act as the + identity represented by the (non-empty) string. This response is + empty when the client is requesting to act as the identity the server + associated with its authentication credentials. + + The syntax of the initial response is specified as a value of the + <extern-initial-resp> production detailed below using the Augmented + Backus-Naur Form (ABNF) [RFC4234] notation. + + external-initial-resp = authz-id-string + authz-id-string = *( UTF8-char-no-nul ) + UTF8-char-no-nul = UTF8-1-no-nul / UTF8-2 / UTF8-3 / UTF8-4 + UTF8-1-no-nul = %x01-7F + + + + +Melnikov & Zeilenga Standards Track [Page 29] + +RFC 4422 SASL June 2006 + + + where the <UTF8-2>, <UTF8-3>, and <UTF8-4> productions are as defined + in [RFC3629]. + + There are no additional challenges and responses. + + Hence, the server is to return the outcome of the authentication + exchange. + + The exchange fails if + + - the client has not established its credentials via external means, + + - the client's credentials are inadequate, + + - the client provided an empty authorization identity string and the + server is unwilling or unable to associate an authorization + identity with the client's credentials, + + - the client provided a non-empty authorization identity string that + is invalid per the syntax requirements of the applicable + application protocol specification, + + - the client provided a non-empty authorization identity string + representing an identity that the client is not allowed to act as, + or + + - the server is unwilling or unable to provide service to the client + for any other reason. + + Otherwise the exchange is successful. When indicating a successful + outcome, additional data is not provided. + +A.2. SASL EXTERNAL Examples + + This section provides examples of EXTERNAL authentication exchanges. + The examples are intended to help the readers understand the above + text. The examples are not definitive. The Application + Configuration Access Protocol (ACAP) [RFC2244] is used in the + examples. + + The first example shows use of EXTERNAL with an empty authorization + identity. In this example, the initial response is not sent in the + client's request to initiate the authentication exchange. + + S: * ACAP (SASL "DIGEST-MD5") + C: a001 STARTTLS + S: a001 OK "Begin TLS negotiation now" + <TLS negotiation, further commands are under TLS layer> + + + +Melnikov & Zeilenga Standards Track [Page 30] + +RFC 4422 SASL June 2006 + + + S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL") + C: a002 AUTHENTICATE "EXTERNAL" + S: + "" + C: + "" + S: a002 OK "Authenticated" + + The second example shows use of EXTERNAL with an authorization + identity of "fred@example.com". In this example, the initial + response is sent with the client's request to initiate the + authentication exchange. This saves a round-trip. + + S: * ACAP (SASL "DIGEST-MD5") + C: a001 STARTTLS + S: a001 OK "Begin TLS negotiation now" + <TLS negotiation, further commands are under TLS layer> + S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL") + C: a002 AUTHENTICATE "EXTERNAL" {16+} + C: fred@example.com + S: a002 NO "Cannot assume requested authorization identity" + +A.3. Security Considerations + + The EXTERNAL mechanism provides no security protection; it is + vulnerable to spoofing by either client or server, active attack, and + eavesdropping. It should only be used when adequate security + services have been established. + +Appendix B. Changes since RFC 2222 + + This appendix is non-normative. + + The material in RFC 2222 was significantly rewritten in the + production of this document. + + RFC 2222, by not stating that the authorization identity string was a + string of Unicode characters, let alone character data, implied that + the authorization identity string was a string of octets. + + - The authorization identity string is now defined as a string of + Unicode characters. The NUL (U+0000) character is prohibited. + While protocol specifications are responsible for defining the + authorization identity form, as well as the Unicode string syntax + and related semantics, mechanism specifications are responsible + for defining how the Unicode string is carried in the + authentication exchange. + + - Deleted "If so, when the client does not send data first, the + initial challenge MUST be specified as being an empty challenge." + + + +Melnikov & Zeilenga Standards Track [Page 31] + +RFC 4422 SASL June 2006 + + + The following technical change was made to the EXTERNAL mechanism: + + - The authorization identity string is to be UTF-8 encoded. + + Note that protocol and mechanism specification requirements have + been significantly tightened. Existing protocol and mechanism + specifications will need to be updated to meet these requirements. + +Editors' Addresses + + Alexey Melnikov + Isode Limited + 5 Castle Business Village + 36 Station Road + Hampton, Middlesex, + TW12 2BX, United Kingdom + + EMail: Alexey.Melnikov@isode.com + URI: http://www.melnikov.ca/ + + + Kurt D. Zeilenga + OpenLDAP Foundation + + EMail: Kurt@OpenLDAP.org + + + + + + + + + + + + + + + + + + + + + + + + + + +Melnikov & Zeilenga Standards Track [Page 32] + +RFC 4422 SASL June 2006 + + +Full Copyright Statement + + Copyright (C) The Internet Society (2006). + + This document is subject to the rights, licenses and restrictions + contained in BCP 78, and except as set forth therein, the authors + retain all their rights. + + This document and the information contained herein are provided on an + "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS + OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET + ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, + INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE + INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED + WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. + +Intellectual Property + + The IETF takes no position regarding the validity or scope of any + Intellectual Property Rights or other rights that might be claimed to + pertain to the implementation or use of the technology described in + this document or the extent to which any license under such rights + might or might not be available; nor does it represent that it has + made any independent effort to identify any such rights. Information + on the procedures with respect to rights in RFC documents can be + found in BCP 78 and BCP 79. + + Copies of IPR disclosures made to the IETF Secretariat and any + assurances of licenses to be made available, or the result of an + attempt made to obtain a general license or permission for the use of + such proprietary rights by implementers or users of this + specification can be obtained from the IETF on-line IPR repository at + http://www.ietf.org/ipr. + + The IETF invites any interested party to bring to its attention any + copyrights, patents or patent applications, or other proprietary + rights that may cover technology that may be required to implement + this standard. Please address the information to the IETF at + ietf-ipr@ietf.org. + +Acknowledgement + + Funding for the RFC Editor function is provided by the IETF + Administrative Support Activity (IASA). + + + + + + + +Melnikov & Zeilenga Standards Track [Page 33] + |