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.
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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
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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
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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.
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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.
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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:
[email protected]
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RFC 3207 SMTP Service Extension - Secure SMTP over TLS February 2002
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Acknowledgement
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