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rfc:rfc2554

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"

Myers Standards Track [Page 1] RFC 2554 SMTP Authentication March 1999

 (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.

Myers Standards Track [Page 2] RFC 2554 SMTP Authentication March 1999

       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.

Myers Standards Track [Page 3] RFC 2554 SMTP Authentication March 1999

       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.

Myers Standards Track [Page 4] RFC 2554 SMTP Authentication March 1999

     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

Myers Standards Track [Page 5] RFC 2554 SMTP Authentication March 1999

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.

Myers Standards Track [Page 6] RFC 2554 SMTP Authentication March 1999

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

Myers Standards Track [Page 7] RFC 2554 SMTP Authentication March 1999

 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.

Myers Standards Track [Page 8] RFC 2554 SMTP Authentication March 1999

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.

Myers Standards Track [Page 9] RFC 2554 SMTP Authentication March 1999

 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

Myers Standards Track [Page 10] RFC 2554 SMTP Authentication March 1999

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.

Myers Standards Track [Page 11]

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