rfc3207.txt

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   C: <opens connection>
   S: 220 mail.imc.org SMTP service ready
   C: EHLO mail.example.com
   S: 250-mail.imc.org offers a warm hug of welcome
   S: 250-8BITMIME
   S: 250-STARTTLS
   S: 250 DSN
   C: STARTTLS
   S: 220 Go ahead
   C: <starts TLS negotiation>
   C & S: <negotiate a TLS session>
   C & S: <check result of negotiation>
   C: EHLO mail.example.com
   S: 250-mail.imc.org touches your hand gently for a moment
   S: 250-8BITMIME
   S: 250 DSN

6. 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



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RFC 3207     SMTP Service Extension - Secure SMTP over TLS February 2002


   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 SMTP client and server must check the result of the TLS
   negotiation to see whether an acceptable degree of authentication and
   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-dependent, 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 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.  Another man-in-the-middle attack is
   to allow the server to announce its STARTTLS capability, but to alter
   the client's request to start TLS and the server's response.  In
   order to defend against such attacks both clients and servers MUST be
   able to be configured to require successful TLS negotiation of an
   appropriate cipher suite for selected hosts before messages can be
   successfully transferred.  The additional option of using TLS when
   possible SHOULD also be provided.  An implementation MAY provide the
   ability to record that TLS was used in communicating with a given
   peer and generating a warning if it is not used in a later session.

   If the TLS negotiation fails or if the client receives a 454
   response, the client has to decide what to do next.  There are three
   main choices: go ahead with the rest of the SMTP session, retry TLS
   at a later time, or give up and return the mail to the sender.  If a
   failure or error occurs, the client can assume that the server may be
   able to negotiate TLS in the future, and should try negotiate TLS in
   a later session, until some locally-chosen timeout occurs, at which
   point, the client should return the mail to the sender.  However, if
   the client and server were only using TLS for authentication, the
   client may want to proceed with the SMTP session, in case some of the
   operations the client wanted to perform are accepted by the server
   even if the client is unauthenticated.

   Before the TLS handshake has begun, any protocol interactions are
   performed in the clear and may be modified by an active attacker.



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RFC 3207     SMTP Service Extension - Secure SMTP over TLS February 2002


   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.

7. References

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
               April 2001.

   [RFC2034]  Freed, N., "SMTP Service Extension for Returning Enhanced
               Error Codes", RFC 2034, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2476]  Gellens, R. and J. Klensin, "Message Submission", RFC
               2476, December 1998.

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

   [SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication",
               RFC 2554, March 1999.

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















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RFC 3207     SMTP Service Extension - Secure SMTP over TLS February 2002


Appendix

   This document is a revision of RFC 2487, which is a Proposed
   Standard. The changes from that document are:

   -  Section 5 and 7: More discussion of the man-in-the-middle attacks
   -  Section 5: Additional discussion of when a server should and
      should not advertise the STARTTLS extension
   -  Section 5: Changed the requirements on SMTP clients after
      receiving a 220 response.
   -  Section 5.1: Clarified description of verifying certificates.
   -  Section 5.3: Added the section on "STARTTLS on the Submission
      Port"
   -  Section 6: Bug fix in the example to indicate that the client
      needs to issue a new EHLO command, as already is described in
      section 5.2.
   -  Section 7: Clarification of the paragraph on acceptable degree of
      privacy. Significant change to the discussion of how to avoid a
      man-in-the-middle attack.
   -  Section A: Update reference from RFC 821 to RFC 2821.

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 3207     SMTP Service Extension - Secure SMTP over TLS February 2002


Full Copyright Statement

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Acknowledgement

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



















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