rfc2487.txt

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RFC 2487                 SMTP Service Extension             January 1999


   C & S: <negotiate a TLS session>
   C & S: <check result of negotiation>
   C: <continues by sending an SMTP command>
   . . .

7. Security Considerations

   It should be noted that SMTP is not an end-to-end mechanism. Thus, if
   an SMTP client/server pair decide to add TLS privacy, they are not
   securing the transport from the originating mail user agent to the
   recipient.  Further, because delivery of a single piece of mail may
   go between more than two SMTP servers, adding TLS privacy to one pair
   of servers does not mean that the entire SMTP chain has been made
   private. Further, just because an SMTP server can authenticate an
   SMTP client, it does not mean that the mail from the SMTP client was
   authenticated by the SMTP client when the client received it.

   Both the STMP client and server must check the result of the TLS
   negotiation to see whether acceptable authentication or privacy was
   achieved. Ignoring this step completely invalidates using TLS for
   security.  The decision about whether acceptable authentication or
   privacy was achieved is made locally, is implementation-dependant,
   and is beyond the scope of this document.

   The SMTP client and server should note carefully the result of the
   TLS negotiation. If the negotiation results in no privacy, or if it
   results in privacy using algorithms or key lengths that are deemed
   not strong enough, or if the authentication is not good enough for
   either party, the client may choose to end the SMTP session with an
   immediate QUIT command, or the server may choose to not accept any
   more SMTP commands.

   A server announcing in an EHLO response that it uses a particular TLS
   protocol should not pose any security issues, since any use of TLS
   will be at least as secure as no use of TLS.

   A man-in-the-middle attack can be launched by deleting the "250
   STARTTLS" response from the server. This would cause the client not
   to try to start a TLS session. An SMTP client can protect against
   this attack by recording the fact that a particular SMTP server
   offers TLS during one session and generating an alarm if it does not
   appear in the EHLO response for a later session. The lack of TLS
   during a session SHOULD NOT result in the bouncing of email, although
   it could result in delayed processing.







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RFC 2487                 SMTP Service Extension             January 1999


   Before the TLS handshake 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 TLS handshake upon completion of the TLS
   handshake.

   The STARTTLS extension is not suitable for authenticating the author
   of an email message unless every hop in the delivery chain, including
   the submission to the first SMTP server, is authenticated. Another
   proposal [SMTP-AUTH] can be used to authenticate delivery and MIME
   security multiparts [MIME-SEC] can be used to authenticate the author
   of an email message. In addition, the [SMTP-AUTH] proposal offers
   simpler and more flexible options to authenticate an SMTP client and
   the SASL EXTERNAL mechanism [SASL] MAY be used in conjunction with
   the STARTTLS command to provide an authorization identity.




































Hoffman                     Standards Track                     [Page 6]

RFC 2487                 SMTP Service Extension             January 1999


A. References

   [RFC-821]   Postel, J., "Simple Mail Transfer Protocol", RFC 821,
               August 1982.

   [RFC-1869]  Klensin, J., Freed, N, Rose, M, Stefferud, E. and D.
               Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
               November 1995.

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

   [RFC-2119]  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.

   [SMTP-AUTH] "SMTP Service Extension for Authentication", Work in
               Progress.

   [TLS]       Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
               RFC 2246, January 1999.

B. Author's Address

   Paul Hoffman
   Internet Mail Consortium
   127 Segre Place
   Santa Cruz, CA  95060

   Phone: [phone]
   EMail: [e-mail]


















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RFC 2487                 SMTP Service Extension             January 1999


C.  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.
























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