SMTP Service Extension for Authentication
draft-siemborski-rfc2554bis-09

Network Working Group                                  Robert Siemborski
INTERNET-DRAFT                                              Google, Inc.
Intended status: Standards Track                         Alexey Melnikov
Obsoletes: RFC 2554 (if approved)                          Isode Limited
Updates: RFC 3463                                             April 2007
Expires: October 2007

               SMTP Service Extension for Authentication
                  <draft-siemborski-rfc2554bis-09.txt>

Status of this Memo
    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
    have been or will be disclosed, and any of which he or she becomes
    aware will be disclosed, in accordance with Section 6 of BCP 79.

    Internet-Drafts are working documents of the Internet
    Engineering Task Force (IETF), its areas, and its
    working groups. Note that other groups may also distribute
    working documents as Internet-Drafts.

    Internet-Drafts are draft documents valid for a maximum of
    six months and may be updated, replaced, or obsoleted by
    other documents at any time. It is inappropriate to use
    Internet-Drafts as reference material or to cite them other
    than as "work in progress."

    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/1id-abstracts.html

    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html

    Distribution of this memo is unlimited.

Copyright Notice

    Copyright (C) The IETF Trust (2007).

Siemborski & Melnikov     Expires October 2007                  [Page 1]
SMTP Service Extension for Authentication                     April 2007

Abstract

    This document defines a Simple Mail Transport Protocol (SMTP)
    extension whereby an SMTP client may indicate an authentication
    mechanism to the server, perform an authentication protocol
    exchange, and optionally negotiate a security layer for subsequent
    protocol interactions during this session.  This extension includes
    a profile of the Simple Authentication and Security Layer (SASL) for
    SMTP.

    This document obsoletes RFC 2554.

Siemborski & Melnikov     Expires October 2007                  [Page 2]
SMTP Service Extension for Authentication                     April 2007

1.  Introduction

    This document defines a Simple Mail Transport Protocol (SMTP)
    extension whereby an SMTP client may indicate an authentication
    mechanism to the server, perform an authentication protocol
    exchange, optionally negotiate a security layer for subsequent
    protocol interactions during this session and, during a mail
    transaction, optionally specify a mailbox associated with the
    identity which submitted the message to the mail delivery system.

    This extension includes a profile of the Simple Authentication and
    Security Layer (SASL) for SMTP.

    When compared to RFC 2554, this document deprecates use of the 538
    response code, adds a new Enhanced Status Code, adds a requirement
    to support SASLprep profile for preparing authorization identities,
    recommends use of RFC 3848 transmission types in the Received trace
    header field, and clarifies interaction with SMTP PIPELINING
    [PIPELINING] extension.

2.  How to Read This Document

    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 [KEYWORDS].

    In examples, "C:" and "S:" indicate lines sent by the client and
    server, respectively.

3.  The Authentication Service Extension

     1.   The name of this [SMTP] service extension is "Authentication"

     2.   The EHLO keyword value associated with this extension is
          "AUTH"

     3.   The AUTH EHLO keyword contains as a parameter a space
          separated list of the names of available [SASL] mechanisms.
          The list of available mechanisms MAY change after a successful
          STARTTLS command [SMTP-TLS].

     4.   A new [SMTP] verb "AUTH" is defined.

     5.   An optional parameter using the keyword "AUTH" is added to the
          MAIL FROM command, and extends the maximum line length of the
          MAIL FROM command by 500 characters.

Siemborski & Melnikov     Expires October 2007                  [Page 3]
SMTP Service Extension for Authentication                     April 2007

     6.   This extension is appropriate for the submission protocol
          [SUBMIT].

4.  The AUTH Command

    AUTH mechanism [initial-response]

      Arguments:
          mechanism: A string identifying a [SASL] authentication
          mechanism.

          initial-response: An optional initial client response.  If
          present, this response MUST be encoded as described in Section
          4 of [BASE64] or contain a single character "=".

      Restrictions:
          After an AUTH command has been successfully completed, no more
          AUTH commands may be issued in the same session.  After a
          successful AUTH command completes, a server MUST reject any
          further AUTH commands with a 503 reply.

          The AUTH command is not permitted during a mail transaction.
          An AUTH command issued during a mail transaction MUST be
          rejected with a 503 reply.

      Discussion:
          The AUTH command initiates a [SASL] authentication exchange
          between the client and the server.  The client identifies the
          SASL mechanism to use with the first parameter of the AUTH
          command.  If the server supports the requested authentication
          mechanism, it performs the SASL exchange to authenticate the
          user.  Optionally, it also negotiates a security layer for
          subsequent protocol interactions during this session.  If the
          requested authentication mechanism is invalid (e.g. is not
          supported or requires an encryption layer), the server rejects
          the AUTH command with a 504 reply, and if it supports the
          [ESMTP-CODES] extension it SHOULD return a 5.5.4 enhanced
          response code.

          The SASL authentication exchange consists of a series of
          server challenges and client responses that are specific to
          the chosen [SASL] mechanism.

          A server challenge is sent as a 334 reply with the text part
          containing the [BASE64] encoded string supplied by the SASL
          mechanism.  This challenge MUST NOT contain any text other
          than the BASE64 encoded challenge.

Siemborski & Melnikov     Expires October 2007                  [Page 4]
SMTP Service Extension for Authentication                     April 2007

          A client response consists of a line containing a [BASE64]
          encoded string.  If the client wishes to cancel the
          authentication exchange, it issues a line with a single "*".
          If the server receives such a response, it MUST reject the
          AUTH command by sending a 501 reply.

          The optional initial response argument to the AUTH command is
          used to save a round trip when using authentication mechanisms
          that support an initial client response.  If the initial
          response argument is omitted and the chosen mechanism requires
          an initial client response, the server MUST proceed as defined
          in section 5.1 of [SASL].  In SMTP, a server challenge that
          contains no data is defined as a 334 reply with no text part.
          Note that there is still a space following the reply code, so
          the complete response line is "334 ".

          Note that the AUTH command is still subject to the line length
          limitations defined in [SMTP]. If use of the initial response
          argument would cause the AUTH command to exceed this length,
          the client MUST NOT use the initial response parameter (and
          instead proceed as defined in Section 5.1 of [SASL]).

          If the client is transmitting an initial response of zero
          length, it MUST instead transmit the response as a single
          equals sign ("=").  This indicates that the response is
          present, but contains no data.

          If the client uses an initial-response argument to the AUTH
          command with a SASL mechanism in which the client does not
          begin the authentication exchange, the server MUST reject the
          AUTH command with a 501 reply.  Servers using the enhanced
          status codes extension [ESMTP-CODES] SHOULD return an enhanced
          status code of 5.7.0 in this case.

          If the server cannot [BASE64] decode any client response, it
          MUST reject the AUTH command with a 501 reply (and an enhanced
          status code of 5.5.2).  If the client cannot BASE64 decode any
          of the server's challenges, it MUST cancel the authentication
          using the "*" response.  In particular, servers and clients
          MUST reject (and not ignore) any character not explicitly
          allowed by the BASE64 alphabet, and MUST reject any sequence
          of BASE64 characters that contains the pad character ('=')
          anywhere other than the end of the string (e.g. "=AAA" and
          "AAA=BBB" are not allowed).

          Note that these [BASE64] strings can be much longer than
          normal SMTP commands.  Clients and servers MUST be able to
          handle the maximum encoded size of challenges and responses

Siemborski & Melnikov     Expires October 2007                  [Page 5]
SMTP Service Extension for Authentication                     April 2007

          generated by their supported authentication mechanisms.  This
          requirement is independent of any line length limitations the
          client or server may have in other parts of its protocol
          implementation.  (At the time of writing of this document,
          12288 is considered to be sufficiently big line length limit
          for handling of deployed authentication mechanisms.)  If,
          during an authentication exchange, the server receives a line
          that is longer than the server's authentication buffer, the
          server fails the AUTH command with the 500 reply. Servers
          using the enhanced status codes extension [ESMTP-CODES] SHOULD
          return an enhanced status code of 5.5.6 in this case.

          The authorization identity generated by this [SASL] exchange
          is a "simple username" (in the sense defined in [SASLprep]),
          and both client and server SHOULD (*) use the [SASLprep]
          profile of the [StringPrep] algorithm to prepare these names
          for transmission or comparison.  If preparation of the
          authorization identity fails or results in an empty string
          (unless it was transmitted as the empty string), the server
          MUST fail the authentication.

          (*) - Future revision of this specification may change this
          requirement to MUST. Currently the SHOULD is used in order to
          avoid breaking the majority of existing implementations.

          If the server is unable to authenticate the client, it SHOULD
          reject the AUTH command with a 535 reply unless a more
          specific error code is appropriate.  Should the client
          successfully complete the exchange, the SMTP server issues a
          235 reply.  (Note that SMTP protocol doesn't support SASL
          feature for returning additional data with the successful
          outcome.)  These status codes, along with others defined by
          this extension, are discussed in Section 6 of this document.

          If a security layer is negotiated during the SASL exchange, it
          takes effect for the client on the octet immediately following
          the CRLF that concludes the last response generated by the
          client.  For the server, it takes effect immediately following
          the CRLF of its success reply.

          When a security layer takes effect, the SMTP protocol is reset
          to the initial state (the state in SMTP after a server issues
          a 220 service ready greeting).  The server MUST discard any
          knowledge obtained from the client, such as the EHLO argument,
          which was not obtained from the SASL negotiation itself.
          Likewise, the client MUST discard any knowledge obtained from
          the server, such as the list of SMTP service extensions, which
          was not obtained from the SASL negotiation itself (Note that a

Siemborski & Melnikov     Expires October 2007                  [Page 6]
SMTP Service Extension for Authentication                     April 2007

          client MAY compare the advertised SASL mechanisms before and
          after authentication in order to detect an active down-
          negotiation attack).

          The client SHOULD send an EHLO command as the first command
          after a successful SASL negotiation which results in the
          enabling of a security layer.

          When both [TLS] and SASL security layers are in effect, when
          sending data the TLS encoding MUST be applied after the SASL
          encoding, regardless of the order in which the layers were
          negotiated.

          The service name specified by this protocol's profile of SASL
          is "smtp".  This service name is also to be used for the
          [SUBMIT] protocol.

          If an AUTH command fails, the client MAY proceed without
          authentication, Alternatively, the client MAY try another
          authentication mechanism or present different credentials by
          issuing another AUTH command.

          Note: a server implementation MUST implement a configuration
          in which it does NOT permit any plaintext password mechanisms,
          unless either the STARTTLS [SMTP-TLS] command has been
          negotiated or some other mechanism that protects the session
          from password snooping has been provided.  Server sites SHOULD
          NOT use any configuration which permits a plaintext password
          mechanism without such a protection mechanism against password
          snooping.

          To ensure interoperability, client and server implementations
          of this extension MUST implement the [PLAIN] SASL mechanism
          running over TLS [TLS] [SMTP-TLS]. See also section 15 for
          additional requirements on implementations of [PLAIN] over
          [TLS].

          Note that many existing client and server implementations
          implement CRAM-MD5 [CRAM-MD5] SASL mechanism. In order to
          ensure interoperability with deployed software new
          implementations MAY implement it, however implementations
          should be aware that this SASL mechanism doesn't provide any
          server authentication. Note that at the time of writing of
          this document the SASL Working Group is working on several
          replacement SASL mechanisms that provide server authentication
          and other features.

          When the AUTH command is used together with the [PIPELINING]

Siemborski & Melnikov     Expires October 2007                  [Page 7]
SMTP Service Extension for Authentication                     April 2007

          extension, it MUST be the last command in a pipelined group of
          commands.  The only exception to this rule is when the AUTH
          command contains an initial response for a SASL mechanism that
          allows client to send data first and is known to complete in
          one round-trip. Two examples of such SASL mechanisms are PLAIN
          [PLAIN] and EXTERNAL [SASL].

4.1.  Examples

    Here is an example of a client attempting AUTH using the [PLAIN]
    SASL mechanism under a TLS layer, and making use of the initial
    client response:

     S: 220-smtp.example.com ESMTP Server
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com
     S: 250-AUTH GSSAPI DIGEST-MD5
     S: 250-ENHANCEDSTATUSCODES
     S: 250 STARTTLS
     C: STARTTLS
     S: 220 Ready to start TLS
       ... TLS negotiation proceeds, further commands
           protected by TLS layer ...
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com
     S: 250 AUTH GSSAPI DIGEST-MD5 PLAIN
     C: AUTH PLAIN dGVzdAB0ZXN0ADEyMzQ=
     S: 235 2.7.0 Authentication successful

    Here is another client that is attempting AUTH PLAIN under a TLS
    layer, this time without the initial response.  Parts of the
    negotiation before the TLS layer was established have been omitted:

       ... TLS negotiation proceeds, further commands
           protected by TLS layer ...
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com
     S: 250 AUTH GSSAPI DIGEST-MD5 PLAIN
     C: AUTH PLAIN
      (note: there is a single space following the 334
       on the following line)
     S: 334
     C: dGVzdAB0ZXN0ADEyMzQ=
     S: 235 2.7.0 Authentication successful

    Here is an example using CRAM-MD5 [CRAM-MD5], a mechanism in which
    the client does not begin the authentication exchange, and includes

Siemborski & Melnikov     Expires October 2007                  [Page 8]
SMTP Service Extension for Authentication                     April 2007

    a server challenge:

     S: 220-smtp.example.com ESMTP Server
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com
     S: 250-AUTH DIGEST-MD5 CRAM-MD5
     S: 250-ENHANCEDSTATUSCODES
     S: 250 STARTTLS
     C: AUTH CRAM-MD5
     S: 334 PDQxOTI5NDIzNDEuMTI4Mjg0NzJAc291cmNlZm91ci5hbmRyZXcuY211LmVk
        dT4=
     C: cmpzMyBlYzNhNTlmZWQzOTVhYmExZWM2MzY3YzRmNGI0MWFjMA==
     S: 235 2.7.0 Authentication successful

    Here is an example of a client attempting AUTH EXTERNAL under TLS,
    using the derived authorization ID (and thus a zero-length initial
    client response).

     S: 220-smtp.example.com ESMTP Server
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com
     S: 250-AUTH GSSAPI DIGEST-MD5
     S: 250-ENHANCEDSTATUSCODES
     S: 250 STARTTLS
     C: STARTTLS
     S: 220 Ready to start TLS
       ... TLS negotiation proceeds, further commands
           protected by TLS layer ...
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com
     S: 250 AUTH EXTERNAL GSSAPI DIGEST-MD5 PLAIN
     C: AUTH EXTERNAL =
     S: 235 2.7.0 Authentication successful

5.  The AUTH Parameter to the MAIL FROM command

    AUTH=mailbox

    Arguments:
        A <mailbox> (see section 4.1.2 of [SMTP]) that is associated
        with the identity which submitted the message to the delivery
        system, or the two character sequence "<>" indicating such an
        identity is unknown or insufficiently authenticated.  To comply
        with restrictions imposed on ESMTP parameters, the <mailbox> is
        encoded inside an xtext.  The syntax of an xtext is described in
        Section 4 of [ESMTP-DSN].

Siemborski & Melnikov     Expires October 2007                  [Page 9]
SMTP Service Extension for Authentication                     April 2007

    Note:
        For the purposes of this discussion, "authenticated identity"
        refers to the identity (if any) derived from the authorization
        identity of previous AUTH command, while the terms "authorized
        identity" and "supplied <mailbox>" refer to the sender identity
        that is being associated with a particular message.  Note that
        one authenticated identity may be able to identify messages as
        being sent by any number of authorized identities within a
        single session.  For example, this may be the case when an SMTP
        server (one authenticated identity) is processing its queue
        (many messages with distinct authorized identities).

    Discussion:
        The optional AUTH parameter to the MAIL FROM command allows
        cooperating agents in a trusted environment to communicate the
        authorization identity associated with individual messages.

        If the server trusts the authenticated identity of the client to
        assert that the message was originally submitted by the supplied
        <mailbox>, then the server SHOULD supply the same <mailbox> in
        an AUTH parameter when relaying the message to any other server
        which supports the AUTH extension.

        For this reason, servers that advertise support for this
        extension MUST support the AUTH parameter to the MAIL FROM
        command even when the client has not authenticated itself to the
        server.

        A MAIL FROM parameter of AUTH=<> indicates that the original
        submitter of the message is not known.  The server MUST NOT
        treat the message as having been originally submitted by
        authenticated identity which resulted from the AUTH command.

        If the AUTH parameter to the MAIL FROM command is not supplied,
        the client has authenticated, and the server believes the
        message is an original submission, the server MAY generate a
        <mailbox> from the user's authenticated identity for use in an
        AUTH parameter when relaying the message to any server which
        supports the AUTH extension.  The generated <mailbox> is
        implementation specific, but it MUST conform to the syntax of
        [SMTP].  If the implementation cannot generate a valid
        <mailbox>, it MUST transmit AUTH=<> when relaying this message.

        If the server does not sufficiently trust the authenticated
        identity of the client, or if the client is not authenticated,
        then the server MUST behave as if the AUTH=<> parameter was
        supplied.  The server MAY, however, write the value of any
        supplied AUTH parameter to a log file.

Siemborski & Melnikov     Expires October 2007                 [Page 10]
SMTP Service Extension for Authentication                     April 2007

        If an AUTH=<> parameter was supplied, either explicitly or due
        to the requirement in the previous paragraph, then the server
        MUST supply the AUTH=<> parameter when relaying the message to
        any server which it has authenticated to using the AUTH
        extension.

        A server MAY treat expansion of a mailing list as a new
        submission, setting the AUTH parameter to the mailing list
        address or mailing list administration address when relaying the
        message to list subscribers.

        Note that an implementation which is hard-coded to treat all
        clients as being insufficiently trusted is compliant with this
        specification.  In that case, the implementation does nothing
        more than parse and discard syntactically valid AUTH parameters
        to the MAIL FROM command, and supply AUTH=<> parameters to any
        servers which it authenticates to.

5.1.  Examples

    An example where the original identity of the sender is trusted and
    known:

     C: MAIL FROM:<e=mc2@example.com> AUTH=e+3Dmc2@example.com
     S: 250 OK

    One example where the identity of the sender is not trusted or is
    otherwise being suppressed by the client:

     C: MAIL FROM:<john+@example.org> AUTH=<>
     S: 250 OK

6.  Status Codes

    The following error codes may be used to indicate various success or
    failure conditions.  Servers that return enhanced status codes
    [ESMTP-CODES] SHOULD use the enhanced codes suggested here.

    235 2.7.0 Authentication Succeeded

    This response to the AUTH command indicates that the authentication
    was successful.

    432 4.7.12 A password transition is needed

    This response to the AUTH command indicates that the user needs to

Siemborski & Melnikov     Expires October 2007                 [Page 11]
SMTP Service Extension for Authentication                     April 2007

    transition to the selected authentication mechanism.  This is
    typically done by authenticating once using the [PLAIN]
    authentication mechanism.  The selected mechanism SHOULD then work
    for authentications in subsequent sessions.

    454 4.7.0 Temporary authentication failure

    This response to the AUTH command indicates that the authentication
    failed due to a temporary server failure.  The client SHOULD NOT
    prompt the user for another password in this case, and instead
    notify the user of server failure.

    534 5.7.9 Authentication mechanism is too weak

    This response to the AUTH command indicates that the selected
    authentication mechanism is weaker than server policy permits for
    that user.  The client SHOULD retry with a new authentication
    mechanism.

    535 5.7.8 Authentication credentials invalid

    This response to the AUTH command indicates that the authentication
    failed due to invalid or insufficient authentication credentials.
    In this case, the client SHOULD ask the user to supply new
    credentials (such as by presenting a password dialog box).

    500 5.5.6 Authentication Exchange line is too long

    This response to the AUTH command indicates that the authentication
    failed due to the client sending a [BASE64] response which is longer
    than the maximum buffer size available for the currently selected
    SASL mechanism.

    530 5.7.0 Authentication required

    This response SHOULD be returned by any command other than AUTH,
    EHLO, HELO, NOOP, RSET, or QUIT when server policy requires
    authentication in order to perform the requested action and
    authentication is not currently in force.

    538 5.7.11 Encryption required for requested authentication
    mechanism

    This response to the AUTH command indicates that the selected
    authentication mechanism may only be used when the underlying SMTP
    connection is encrypted. Note that this response code is documented
    here for historical purposes only.  Modern implementations SHOULD
    NOT advertise mechanisms that are not permitted due to lack of

Siemborski & Melnikov     Expires October 2007                 [Page 12]
SMTP Service Extension for Authentication                     April 2007

    encryption, unless an encryption layer of sufficient strength is
    currently being employed.

    This document adds several new enhanced status code to the list
    defined in [ENHANCED]:

    The following 3 Enhanced Status Codes were defined above:

     5.7.8 Authentication credentials invalid
     5.7.9 Authentication mechanism is too weak
     5.7.11 Encryption required for requested authentication mechanism

    X.5.6     Authentication Exchange line is too long

    This enhanced status code SHOULD be returned when the server fails
    the AUTH command due to the client sending a [BASE64] response which
    is longer than the maximum buffer size available for the currently
    selected SASL mechanism. This is useful for both permanent and
    persistent transient errors.

7.  Additional requirements on servers

    As described in Section 4.4 of [SMTP], an SMTP server that receives
    a message for delivery or further processing MUST insert the
    "Received:" header field at the beginning of the message content.
    This document places additional requirements on the content of a
    generated "Received:" header field. Upon successful authentication a
    server SHOULD use the "ESMTPA" or the "ESMTPSA" [SMTP-TT] (when
    appropriate) keyword in the "with" clause of the Received header
    field.

8.  Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form notation as specified in [ABNF].  Non-terminals referenced but
    not defined below are as defined by [ABNF] or [SASL]. The non-
    terminal <mailbox> is defined in [SMTP].

    Except as noted otherwise, all alphabetic characters are case-
    insensitive.  The use of upper or lower case characters to define
    token strings is for editorial clarity only.  Implementations MUST
    accept these strings in a case-insensitive fashion.

        hexchar         = "+" HEXDIG HEXDIG

Siemborski & Melnikov     Expires October 2007                 [Page 13]
SMTP Service Extension for Authentication                     April 2007

        xchar           = %x21-2A / %x2C-3C / %x3E-7E
                          ;; US-ASCII except for "+", "=", SP and CTL

        xtext           = *(xchar / hexchar)
                          ;; non-US-ASCII is only allowed as hexchar

        auth-command    = "AUTH" SP sasl-mech [SP initial-response]
                          *(CRLF [base64]) [CRLF cancel-response]
                          CRLF
                          ;; <sasl-mech> is defined in [SASL]

        auth-param      = "AUTH=" xtext
                            ;; Parameter to the MAIL FROM command.
                            ;; This non-terminal complies with
                            ;; syntax defined by esmtp-param [SMTP].
                            ;;
                            ;; The decoded form of the xtext MUST be
                            ;; either a <mailbox> or the two
                            ;; characters "<>"

        base64          = base64-terminal /
                          ( 1*(4base64-char) [base64-terminal] )

        base64-char     = ALPHA / DIGIT / "+" / "/"
                          ;; Case-sensitive

        base64-terminal = (2base64-char "==") / (3base64-char "=")

        continue-req    = "334" SP [base64] CRLF
                            ;; Intermediate response to the AUTH
                            ;; command.
                            ;; This non-terminal complies with
                            ;; syntax defined by Reply-line [SMTP].

        initial-response= base64 / "="

        cancel-response = "*"

9.  Security Considerations

    Security issues are discussed throughout this memo.

    If a client uses this extension to get an encrypted tunnel through
    an insecure network to a cooperating server, it needs to be
    configured to never send mail to that server when the connection is
    not mutually authenticated and encrypted.  Otherwise, an attacker

Siemborski & Melnikov     Expires October 2007                 [Page 14]
SMTP Service Extension for Authentication                     April 2007

    could steal the client's mail by hijacking the [SMTP] connection and
    either pretending the server does not support the Authentication
    extension or causing all AUTH commands to fail.

    Before the [SASL] negotiation has begun, any protocol interactions
    are performed in the clear and may be modified by an active
    attacker.  For this reason, clients and servers MUST discard any
    knowledge obtained prior to the start of the SASL negotiation upon
    the establishment of a security layer.

    This mechanism does not protect the TCP port, so an active attacker
    may redirect a relay connection attempt (i.e. a connection between
    two MTAs) to the submission port [SUBMIT].  The AUTH=<> parameter
    prevents such an attack from causing a relayed message, in the
    absence of other envelope authentication, from picking up the
    authentication of the relay client.

    A message submission client may require the user to authenticate
    whenever a suitable [SASL] mechanism is advertised.  Therefore, it
    may not be desirable for a submission server [SUBMIT] to advertise a
    SASL mechanism when use of that mechanism grants the clients no
    benefits over anonymous submission.

    Servers MAY implement a policy whereby the connection is dropped
    after a number of failed authentication attempts.  If they do so,
    they SHOULD NOT drop the connection until at least 3 attempts to
    authenticate have failed.

    If an implementation supports SASL mechanisms that are vulnerable to
    passive eavesdropping attacks (such as [PLAIN]), then the
    implementation MUST support at least one configuration where these
    SASL mechanisms are not advertised or used without the presence of
    an external security layer such as [TLS].

    This extension is not intended to replace or be used instead of end-
    to-end message signature and encryption systems such as [S/MIME] or
    [PGP].  This extension addresses a different problem than end-to-end
    systems; it has the following key differences:

     1.   It is generally useful only within a trusted enclave.

     2.   It protects the entire envelope of a message, not just the
          message's body.

     3.   It authenticates the message submission, not authorship of the
          message content.

Siemborski & Melnikov     Expires October 2007                 [Page 15]
SMTP Service Extension for Authentication                     April 2007

     4.   When mutual authentication is used along with a security
          layer, it can give the sender some assurance that the message
          was successfully delivered to the next hop.

    Additional security considerations are mentioned in the [SASL]
    specification.  Additional security considerations specific to a
    particular SASL mechanism are described in the relevant
    specification.  Additional security considerations for [PLAIN] over
    [TLS] are mentioned in Section 15 of this document.

10.  IANA Considerations

    This document requests that the IANA update the entry for the "smtp"
    SASL protocol name to point at this document.

    This document requests that the IANA updates registration of the
    Authentication SMTP service extension as defined in Section 3 of
    this document. This registry is currently located at
    <http://www.iana.org/assignments/mail-parameters>.

11.  Normative References

[ABNF]      Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 4234, October 2005.

[BASE64]    Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 4648, October 2006.

[ESMTP-CODES]
            Freed, N., "SMTP Service Extension for Returning Enhanced
            Error Codes", RFC 2034, October 1996.

[ENHANCED]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
            3463, January 2003.

[ESMTP-DSN] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
            Extension Delivery Status Notifications (DSNs)", RFC 3461,
            January 2003.

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

[SASL]      Melnikov, A. and K. Zeilenga, "Simple Authentication and
            Security Layer (SASL)", RFC 4422, June 2006.

Siemborski & Melnikov     Expires October 2007                 [Page 16]
SMTP Service Extension for Authentication                     April 2007

[SASLprep]  Zeilega, K., "SASLprep: Stringprep profile for user names
            and passwords", RFC 4013, February 2005.

[SMTP]      Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
            April 2001.

[SMTP-TLS]  Hoffman, P. "SMTP Service Extension for Secure SMTP over
            Transport Layer Security", RFC 3207, February 2002.

[StringPrep]
            Hoffman, P., Blanchet, M., "Preparation of Internationalized
            Strings ("stringprep")", RFC 3454, December 2002.

[SUBMIT]    Gellens, R. and J. Klensin, "Message Submission for Mail",
            RFC 4409, April 2006.

[SMTP-TT]   Newman, C., "ESMTP and LMTP Transmission Types
            Registration", RFC 3848, July 2004.

[PLAIN]     Zeilenga, K. (Ed.), "The PLAIN Simple Authentication and
            Security Layer (SASL) Mechanism", RFC 4616, August 2006.

[X509]      Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509
            Public Key Infrastructure Certificate and Certificate
            Revocation List (CRL) Profile", RFC 3280, April 2002.

12.  Informative References

[PGP]       Elkins, M., "MIME Security with Pretty Good Privacy (PGP)",
            RFC 2015, October 1996.

[S/MIME]    Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
            2633, June 1999.

[TLS]       Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.1", RFC 4346, April 2006.

[PIPELINING]
            Freed, N., "SMTP Service Extension for Command Pipelining",
            RFC 2920, September 2000.

[CRAM-MD5]  Klensin, J., Catoe, R. and P. Krumviede, IMAP/POP AUTHorize
            Extension for Simple Challenge/Response", RFC 2195,
            September 1997.

Siemborski & Melnikov     Expires October 2007                 [Page 17]
SMTP Service Extension for Authentication                     April 2007

13.  Editors' Addresses

    Robert Siemborski
    Google, Inc.
    1600 Ampitheatre Parkway
    Mountain View, CA 94043, USA
    +1 650 623 6925
    robsiemb@google.com

    Alexey Melnikov
    Isode Limited
    5 Castle Business Village, 36 Station Road,
    Hampton, Middlesex, TW12 2BX, UK
    Alexey.Melnikov@isode.com

14.  Acknowledgments:

    Editors would like to acknowledge the contributions of John Myers
    and other contributors to RFC 2554, on which this document draws
    from heavily.

    Editors would also like to thank Ken Murchison, Mark Crispin, Chris
    Newman, David Wilson, Dave Cridland, Frank Ellermann, Ned Freed,
    John Klensin, Tony Finch, Abhijit Menon-Sen, Philip Guenther, Sam
    Hartman, Russ Housley, Cullen Jennings and Lisa Dusseault for the
    time they devoted to reviewing of this document and/or for the
    comments received.

15.  Additional requirements when using SASL PLAIN over TLS

    This section is normative for SMTP implementations that support SASL
    [PLAIN] over [TLS].

    If an SMTP client is willing to use SASL PLAIN over TLS to
    authenticate to the SMTP server, the client verifies the server
    certificate according to the rules of [X509]. If the server has not
    provided any certificate, or if the certificate verification fails,
    the client MUST NOT attempt to authenticate using the SASL PLAIN
    mechanism.

    After a successful [TLS] negotiation, the client MUST check its
    understanding of the server hostname against the server's identity
    as presented in the server Certificate message, in order to prevent
    man-in-the-middle attacks. If the match fails, the client MUST NOT
    attempt to authenticate using the SASL PLAIN mechanism. Matching is
    performed according to the following rules:

        The client MUST use the server hostname it used to open the

Siemborski & Melnikov     Expires October 2007                 [Page 18]
SMTP Service Extension for Authentication                     April 2007

        connection as the value to compare against the server name as
        expressed in the server certificate.  The client MUST NOT use
        any form of the server hostname derived from an insecure remote
        source (e.g., insecure DNS lookup).  CNAME canonicalization is
        not done.

        If a subjectAltName extension of type dNSName is present in the
        certificate, it SHOULD be used as the source of the server's
        identity.

        Matching is case-insensitive.

        A "*" wildcard character MAY be used as the left-most name
        component in the certificate.  For example, *.example.com would
        match a.example.com, foo.example.com, etc. but would not match
        example.com.

        If the certificate contains multiple names (e.g., more than one
        dNSName field), then a match with any one of the fields is
        considered acceptable.

16.  Changes Since RFC 2554

     1.   Clarify that servers MUST support the use of the AUTH=mailbox
          parameter to MAIL FROM, even when the client is not
          authenticated.

     2.   Clarify the initial-client-send requirements, and give
          additional examples.

     3.   Update references to newer versions of various specifications.

     4.   Require SASL PLAIN (over TLS) as mandatory-to-implement.

     5.   Clarify that the mechanism list can change.

     6.   Deprecate the use of the 538 response code.

     7.   Add the use of the SASLprep profile for preparing
          authorization identities.

     8.   Substantial cleanup of response codes and indicate suggested
          enhanced response codes.  Also indicate what response codes
          should result in a client prompting the user for new
          credentials.

Siemborski & Melnikov     Expires October 2007                 [Page 19]
SMTP Service Extension for Authentication                     April 2007

     9.   Updated ABNF section to use RFC 4234.

     10.  Clarified interaction with SMTP PIPELINING extension.

     11.  Added a reference to RFC 3848.

     12.  Added a new Enhanced Status Code for "authentication line too
          long" case.

     13.  General other editorial clarifications.

17.  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.

18.  Full Copyright Statement

    Copyright (C) The IETF Trust (2007).

    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, THE
    IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
    WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY

Siemborski & Melnikov     Expires October 2007                 [Page 20]
SMTP Service Extension for Authentication                     April 2007

    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.

Acknowledgement

    Funding for the RFC Editor function is currently provided by the
    Internet Society.

Siemborski & Melnikov     Expires October 2007                 [Page 21]
SMTP Service Extension for Authentication                     April 2007

                           Table of Contents

1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
2. How to Read This Document . . . . . . . . . . . . . . . . . . . .   3
3. The Authentication Service Extension  . . . . . . . . . . . . . .   3
4. The AUTH Command  . . . . . . . . . . . . . . . . . . . . . . . .   4
4.1. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
5. The AUTH Parameter to the MAIL FROM command . . . . . . . . . . .   9
5.1. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
6. Status Codes  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
7. Additional requirements on servers  . . . . . . . . . . . . . . .  13
8. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  13
9. Security Considerations . . . . . . . . . . . . . . . . . . . . .  14
10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . .  16
11. Normative References . . . . . . . . . . . . . . . . . . . . . .  16
12. Informative References . . . . . . . . . . . . . . . . . . . . .  17
13. Editors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  18
14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  18
15. Additional requirements when using SASL PLAIN over TLS . . . . .  18
16. Changes Since RFC 2554 . . . . . . . . . . . . . . . . . . . . .  19
17. Intellectual Property  . . . . . . . . . . . . . . . . . . . . .  20
18. Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  20

Siemborski & Melnikov     Expires October 2007                  [Page 1]