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RFC 3207 - SMTP Service Extension for Secure SMTP over Transport


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Network Working Group                                         P. Hoffman
Request for Comments: 3207                      Internet Mail Consortium
Obsoletes: 2487                                            February 2002
Category: Standards Track

                      SMTP Service Extension for
               Secure SMTP over Transport Layer Security

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 (2002).  All Rights Reserved.

Abstract

   This document describes an extension to the SMTP (Simple Mail
   Transfer Protocol) service that allows an SMTP server and client to
   use TLS (Transport Layer Security) to provide private, authenticated
   communication over the Internet.  This gives SMTP agents the ability
   to protect some or all of their communications from eavesdroppers and
   attackers.

1. Introduction

   SMTP [RFC2821] servers and clients normally communicate in the clear
   over the Internet.  In many cases, this communication goes through
   one or more router that is not controlled or trusted by either
   entity.  Such an untrusted router might allow a third party to
   monitor or alter the communications between the server and client.

   Further, there is often a desire for two SMTP agents to be able to
   authenticate each others' identities.  For example, a secure SMTP
   server might only allow communications from other SMTP agents it
   knows, or it might act differently for messages received from an
   agent it knows than from one it doesn't know.

   TLS [TLS], more commonly known as SSL, is a popular mechanism for
   enhancing TCP communications with privacy and authentication.  TLS is
   in wide use with the HTTP protocol, and is also being used for adding
   security to many other common protocols that run over TCP.

   This document obsoletes RFC 2487.

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2. STARTTLS Extension

   The STARTTLS extension to SMTP is laid out as follows:

   (1) the name of the SMTP service defined here is STARTTLS;

   (2) the EHLO keyword value associated with the extension is STARTTLS;

   (3) the STARTTLS keyword has no parameters;

   (4) a new SMTP verb, "STARTTLS", is defined;

   (5) no additional parameters are added to any SMTP command.

3. The STARTTLS Keyword

   The STARTTLS keyword is used to tell the SMTP client that the SMTP
   server is currently able to negotiate the use of TLS.  It takes no
   parameters.

4. The STARTTLS Command

   The format for the STARTTLS command is:

   STARTTLS

   with no parameters.

   After the client gives the STARTTLS command, the server responds with
   one of the following reply codes:

   220 Ready to start TLS
   501 Syntax error (no parameters allowed)
   454 TLS not available due to temporary reason

   If the client receives the 454 response, the client must decide
   whether or not to continue the SMTP session.  Such a decision is
   based on local policy.  For instance, if TLS was being used for
   client authentication, the client might try to continue the session,
   in case the server allows it even with no authentication.  However,
   if TLS was being negotiated for encryption, a client that gets a 454
   response needs to decide whether to send the message anyway with no
   TLS encryption, whether to wait and try again later, or whether to
   give up and notify the sender of the error.

   A publicly-referenced SMTP server MUST NOT require use of the
   STARTTLS extension in order to deliver mail locally.  This rule
   prevents the STARTTLS extension from damaging the interoperability of
   the Internet's SMTP infrastructure.  A publicly-referenced SMTP
   server is an SMTP server which runs on port 25 of an Internet host
   listed in the MX record (or A record if an MX record is not present)
   for the domain name on the right hand side of an Internet mail
   address.

   Any SMTP server may refuse to accept messages for relay based on
   authentication supplied during the TLS negotiation.  An SMTP server
   that is not publicly referenced may refuse to accept any messages for
   relay or local delivery based on authentication supplied during the
   TLS negotiation.

   A SMTP server that is not publicly referenced may choose to require
   that the client perform a TLS negotiation before accepting any
   commands.  In this case, the server SHOULD return the reply code:

   530 Must issue a STARTTLS command first

   to every command other than NOOP, EHLO, STARTTLS, or QUIT.  If the
   client and server are using the ENHANCEDSTATUSCODES ESMTP extension
   [RFC2034], the status code to be returned SHOULD be 5.7.0.

   After receiving a 220 response to a STARTTLS command, the client MUST
   start the TLS negotiation before giving any other SMTP commands.  If,
   after having issued the STARTTLS command, the client finds out that
   some failure prevents it from actually starting a TLS handshake, then
   it SHOULD abort the connection.

   If the SMTP client is using pipelining as defined in RFC 2920, the
   STARTTLS command must be the last command in a group.

4.1 Processing After the STARTTLS Command

   After the TLS handshake has been completed, both parties MUST
   immediately decide whether or not to continue based on the
   authentication and privacy achieved.  The SMTP client and server may
   decide to move ahead even if the TLS negotiation ended with no
   authentication and/or no privacy because most SMTP services are
   performed with no authentication and no privacy, but some SMTP
   clients or servers may want to continue only if a particular level of
   authentication and/or privacy was achieved.

   If the SMTP client decides that the level of authentication or
   privacy is not high enough for it to continue, it SHOULD issue an
   SMTP QUIT command immediately after the TLS negotiation is complete.
   If the SMTP server decides that the level of authentication or
   privacy is not high enough for it to continue, it SHOULD reply to
   every SMTP command from the client (other than a QUIT command) with
   the 554 reply code (with a possible text string such as "Command
   refused due to lack of security").

   The decision of whether or not to believe the authenticity of the
   other party in a TLS negotiation is a local matter.  However, some
   general rules for the decisions are:

   -  A SMTP client would probably only want to authenticate an SMTP
      server whose server certificate has a domain name that is the
      domain name that the client thought it was connecting to.
   -  A publicly-referenced  SMTP server would probably want to accept
      any verifiable certificate from an SMTP client, and would possibly
      want to put distinguishing information about the certificate in
      the Received header of messages that were relayed or submitted
      from the client.

4.2 Result of the STARTTLS Command

   Upon completion of the TLS handshake, 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 TLS 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 TLS
   negotiation itself.  The client SHOULD send an EHLO command as the
   first command after a successful TLS negotiation.

   The list of SMTP service extensions returned in response to an EHLO
   command received after the TLS handshake MAY be different than the
   list returned before the TLS handshake.  For example, an SMTP server

   might not want to advertise support for a particular SASL mechanism
   [SASL] unless a client has sent an appropriate client certificate
   during a TLS handshake.

   Both the client and the server MUST know if there is a TLS session
   active.  A client MUST NOT attempt to start a TLS session if a TLS
   session is already active.  A server MUST NOT return the STARTTLS
   extension in response to an EHLO command received after a TLS
   handshake has completed.

4.3 STARTTLS on the Submission Port

   STARTTLS is a valid ESMTP extension when used on the Submission port,
   as defined in [RFC2476].  In fact, since the submission port is by
   definition not a publicly referenced SMTP server, the STARTTLS
   extension can be particularly useful by providing security and
   authentication for this service.

5. Usage Example

   The following dialog illustrates how a client and server can start a
   TLS session:

   S: <waits for connection on TCP port 25>
   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

   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.

   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.

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: (831) 426-9827
   EMail: phoffman@imc.org

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Acknowledgement

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

 

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