Internet RFC/STD/FYI/BCP Archives

# RFC2554

[ Index | Search | What's New | Comments | Help ]

    Network Working Group                                           J. Myers
    Request for Comments: 2554                       Netscape Communications
    Category: Standards Track                                     March 1999

                             SMTP Service Extension
                               for Authentication

    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 (1999).  All Rights Reserved.

    1. Introduction

       This document defines an SMTP service extension [ESMTP] 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.  This
       extension is a profile of the Simple Authentication and Security
       Layer [SASL].

    2. Conventions Used in this Document

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

       The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
       in this document are to be interpreted as defined in "Key words for
       use in RFCs to Indicate Requirement Levels" [KEYWORDS].

    3. The Authentication service extension

       (1) the name of the 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 supported SASL mechanisms.

       (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.

       (6) this extension is appropriate for the submission protocol
           [SUBMIT].

    4. The AUTH command

       AUTH mechanism [initial-response]

         Arguments:
             a string identifying a SASL authentication mechanism.
             an optional base64-encoded response

         Restrictions:
             After an AUTH command has 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.

         Discussion:
             The AUTH command indicates an authentication mechanism to the
             server.  If the server supports the requested authentication
             mechanism, it performs an authentication protocol exchange to
             authenticate and identify the user.  Optionally, it also
             negotiates a security layer for subsequent protocol
             interactions.  If the requested authentication mechanism is not
             supported, the server rejects the AUTH command with a 504
             reply.

             The authentication protocol exchange consists of a series of
             server challenges and client answers that are specific to the
             authentication mechanism.  A server challenge, otherwise known
             as a ready response, is a 334 reply with the text part
             containing a BASE64 encoded string.  The client answer consists
             of a line containing a BASE64 encoded string.  If the client
             wishes to cancel an authentication exchange, it issues a line
             with a single "*".  If the server receives such an answer, 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 are defined to send no data in the initial challenge.
             When the initial-response argument is used with such a
             mechanism, the initial empty challenge is not sent to the
             client and the server uses the data in the initial-response
             argument as if it were sent in response to the empty challenge.
             Unlike a zero-length client answer to a 334 reply, a zero-
             length initial response is sent as a single equals sign ("=").
             If the client uses an initial-response argument to the AUTH
             command with a mechanism that sends data in the initial
             challenge, the server rejects the AUTH command with a 535
             reply.

             If the server cannot BASE64 decode the argument, it rejects the
             AUTH command with a 501 reply.  If the server rejects the
             authentication data, it SHOULD reject the AUTH command with a
             535 reply unless a more specific error code, such as one listed
             in section 6, is appropriate.  Should the client successfully
             complete the authentication exchange, the SMTP server issues a
             235 reply.

             The service name specified by this protocol's profile of SASL
             is "smtp".

             If a security layer is negotiated through the SASL
             authentication exchange, it takes effect immediately following
             the CRLF that concludes the authentication exchange for the
             client, and the CRLF of the success reply for the server.  Upon
             a security layer's taking 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 argument to the
             EHLO command, which was not obtained from the SASL negotiation
             itself.  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 (with the
             exception that a client MAY compare the list of 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.

             The server is not required to support any particular
             authentication mechanism, nor are authentication mechanisms
             required to support any security layers.  If an AUTH command
             fails, the client may try another authentication mechanism by
             issuing another AUTH command.

             If an AUTH command fails, the server MUST behave the same as if
             the client had not issued the AUTH command.

             The BASE64 string may in general be arbitrarily long.  Clients
             and servers MUST be able to support challenges and responses
             that are as long as are generated by the authentication
             mechanisms they support, independent of any line length
             limitations the client or server may have in other parts of its
             protocol implementation.

         Examples:
             S: 220 smtp.example.com ESMTP server ready
             C: EHLO jgm.example.com
             S: 250-smtp.example.com
             S: 250 AUTH CRAM-MD5 DIGEST-MD5
             C: AUTH FOOBAR
             S: 504 Unrecognized authentication type.
             C: AUTH CRAM-MD5
             S: 334
             PENCeUxFREJoU0NnbmhNWitOMjNGNndAZWx3b29kLmlubm9zb2Z0LmNvbT4=
             C: ZnJlZCA5ZTk1YWVlMDljNDBhZjJiODRhMGMyYjNiYmFlNzg2ZQ==
             S: 235 Authentication successful.

    5. The AUTH parameter to the MAIL FROM command

       AUTH=addr-spec

       Arguments:
           An addr-spec containing 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 the restrictions imposed on ESMTP
           parameters, the addr-spec is encoded inside an xtext.  The syntax
           of an xtext is described in section 5 of [ESMTP-DSN].

       Discussion:
           The optional AUTH parameter to the MAIL FROM command allows
           cooperating agents in a trusted environment to communicate the
           authentication of individual messages.

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

           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 the client.

           If the AUTH parameter to the MAIL FROM is not supplied, the
           client has authenticated, and the server believes the message is
           an original submission by the client, the server MAY supply the
           client's identity in the addr-spec in an AUTH parameter when
           relaying the message to any server which supports the AUTH
           extension.

           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 the AUTH
           parameter to a log file.

           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.

           It is conforming for an implementation to be hard-coded to treat
           all clients as being insufficiently trusted.  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 to which it
           authenticates using the AUTH extension.

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

    6. Error Codes

       The following error codes may be used to indicate various conditions
       as described.

       432 A password transition is needed

       This response to the AUTH command indicates that the user needs to
       transition to the selected authentication mechanism.  This typically
       done by authenticating once using the PLAIN authentication mechanism.

       534 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.

       538 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.

       454 Temporary authentication failure

       This response to the AUTH command indicates that the authentication
       failed due to a temporary server failure.

       530 Authentication required

       This response may be returned by any command other than AUTH, EHLO,
       HELO, NOOP, RSET, or QUIT.  It indicates that server policy requires
       authentication in order to perform the requested action.

    7. Formal Syntax

       The following syntax specification uses the augmented Backus-Naur
       Form (BNF) notation as specified in [ABNF].

       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.

       UPALPHA         = %x41-5A            ;; Uppercase: A-Z

       LOALPHA         = %x61-7A            ;; Lowercase: a-z

       ALPHA           = UPALPHA / LOALPHA  ;; case insensitive

       DIGIT           = %x30-39            ;; Digits 0-9

       HEXDIGIT        = %x41-46 / DIGIT    ;; hexidecimal digit (uppercase)

       hexchar         = "+" HEXDIGIT HEXDIGIT

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

       xtext           = *(xchar / hexchar)

       AUTH_CHAR       = ALPHA / DIGIT / "-" / "_"

       auth_type       = 1*20AUTH_CHAR

       auth_command    = "AUTH" SPACE auth_type [SPACE (base64 / "=")]
                         *(CRLF [base64]) CRLF

       auth_param      = "AUTH=" xtext
                           ;; The decoded form of the xtext MUST be either
                           ;; an addr-spec or the two characters "<>"

       base64          = base64_terminal /
                         ( 1*(4base64_CHAR) [base64_terminal] )

       base64_char     = UPALPHA / LOALPHA / DIGIT / "+" / "/"
                         ;; Case-sensitive

       base64_terminal = (2base64_char "==") / (3base64_char "=")

       continue_req    = "334" SPACE [base64] CRLF

       CR              = %x0C           ;; ASCII CR, carriage return

       CRLF            = CR LF

       CTL             = %x00-1F / %x7F ;; any ASCII control character and DEL

       LF              = %x0A           ;; ASCII LF, line feed

       SPACE           = %x20           ;; ASCII SP, space

    8. References

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

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

       [ESMTP]     Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
                   Crocker, "SMTP Service Extensions", RFC 1869, November
                   1995.

       [ESMTP-DSN] Moore, K, "SMTP Service Extension for Delivery Status
                   Notifications", RFC 1891, January 1996.

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

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

       [SUBMIT]    Gellens, R. and J. Klensin, "Message Submission", RFC
                   2476, December 1998.

       [RFC821]    Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
                   821, August 1982.

       [RFC822]    Crocker, D., "Standard for the Format of ARPA Internet
                   Text Messages", STD 11, RFC 822, August 1982.

    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 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 completion
       of a SASL negotiation which results in a security layer.

       This mechanism does not protect the TCP port, so an active attacker
       may redirect a relay connection attempt to the submission port
       [SUBMIT].  The AUTH=<> parameter prevents such an attack from causing
       an relayed message without an envelope authentication to pick 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 client no benefits
       over anonymous submission.

       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

          (4) it can give the sender some assurance the message was
              delivered to the next hop in the case where the sender
              mutually authenticates with the next hop and negotiates an
              appropriate security layer.

       Additional security considerations are mentioned in the SASL
       specification [SASL].

    10. Author's Address

       John Gardiner Myers
       Netscape Communications
       501 East Middlefield Road
       Mail Stop MV-029
       Mountain View, CA 94043

       EMail: jgmyers@netscape.com

    11.  Full Copyright Statement

       Copyright (C) The Internet Society (1999).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS 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.

[ Index | Search | What's New | Comments | Help ]