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Network Working Group                                         P. HoffmanRequest for Comments: 3207                      Internet Mail ConsortiumObsoletes:2487                                            February 2002Category: Standards TrackSMTP Service Extension forSecure SMTP over Transport Layer SecurityStatus 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 obsoletesRFC 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 reasonHoffman                     Standards Track                     [Page 2]

RFC 3207     SMTP Service Extension - Secure SMTP over TLS February 2002   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 inRFC 2920, the   STARTTLS command must be the last command in a group.Hoffman                     Standards Track                     [Page 3]

RFC 3207     SMTP Service Extension - Secure SMTP over TLS February 20024.1 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 serverHoffman                     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 DSN6. 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 pairHoffman                     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",RFC2476, 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 2002Appendix   This document is a revision ofRFC 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 insection 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 fromRFC 821 toRFC 2821.Author's Address   Paul Hoffman   Internet Mail Consortium   127 Segre Place   Santa Cruz, CA  95060   Phone: (831) 426-9827   EMail: phoffman@imc.orgHoffman                     Standards Track                     [Page 8]

RFC 3207     SMTP Service Extension - Secure SMTP over TLS February 2002Full Copyright Statement   Copyright (C) The Internet Society (2002).  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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Hoffman                     Standards Track                     [Page 9]