SMTP AUTH: SMTP Service Extension for Authentication

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

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