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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.

Hoffman Standards Track [Page 1] RFC 3207 SMTP Service Extension - Secure SMTP over TLS February 2002

 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

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 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.

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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

Hoffman Standards Track [Page 4] RFC 3207 SMTP Service Extension - Secure SMTP over TLS February 2002

 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

Hoffman Standards Track [Page 5] 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.

Hoffman Standards Track [Page 6] 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.

Hoffman Standards Track [Page 7] 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: (831) 426-9827
 EMail: phoffman@imc.org

Hoffman Standards Track [Page 8] RFC 3207 SMTP Service Extension - Secure SMTP over TLS February 2002

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 Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Acknowledgement

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

Hoffman Standards Track [Page 9]