STARTTLS for SMTP: SMTP Service Extension for Secure SMTP over TLS

Network Working Group                                     P. Hoffman
Request for Comments: 2487                  Internet Mail Consortium
Category: Standards Track                               January 1999


            SMTP Service Extension for Secure SMTP over TLS

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. Abstract

   This document describes an extension to the SMTP service that allows
   an SMTP server and client to use 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.

2. Introduction

   SMTP [RFC-821] 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.

2.1 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 [RFC-2119].

3. 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.

4. The STARTTLS Keyword

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

5. 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

   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
   [RFC-2034], the status code to be returned SHOULD be 5.7.0.

   After receiving a 220 response to a STARTTLS command, the client
   SHOULD start the TLS negotiation before giving any other SMTP
   commands.

   If the SMTP client is using pipelining as defined in RFC 1854, the
   STARTTLS command must be the last command in a group.

5.1 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 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.

5.2 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 TLS extension
   in response to an EHLO command received after a TLS handshake has
   completed.

6. 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.ietf.org
   S: 250-mail.imc.org offers a warm hug of welcome
   S: 250 STARTTLS
   C: STARTTLS
   S: 220 Go ahead
   C: <starts TLS negotiation>
   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.

   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.

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

C.  Full Copyright Statement

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