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.
AbstractThis 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. 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. RFC2119].
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.
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. 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.
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. [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.
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.
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