Network Working Group Robert Siemborski
INTERNET-DRAFT Carnegie Mellon University
Intended Category: Proposed Standard October, 2003
SMTP Service Extension for Authentication
<draft-siemborski-rfc2554bis-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Distribution of this memo is unlimited.
Abstract
This document defines a Simple Mail Transport Protocol (SMTP)
extension whereby an SMTP client may indicate an authentication
mechanism to the server, perform an authentication protocol
exchange, and optionally negotiate a security layer for subsequent
protocol interactions during this session. This extension includes
a profile of the Simple Authentication and Security Layer (SASL) for
SMTP.
This document obsoletes RFC 2554 and replaces it as a Proposed
Standard.
1. How to Read This Document
The key words "MUST, "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", and "MAY" in this document are to be interpreted as
defined in "Key words for use in RFCs to Indicate Requirement
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Table of Contents
1. How to Read This Document . . . . . . . . . . . . . . . . . . . . 1
2. The Authentication Service Extension . . . . . . . . . . . . . . 3
3. The AUTH Command . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. The AUTH Parameter to the MAIL FROM command . . . . . . . . . . . 7
4.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
5. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 12
9. Intellectual Property Rights . . . . . . . . . . . . . . . . . . 12
10. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12. Changes Since RFC 2554 . . . . . . . . . . . . . . . . . . . . . 15
13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 15
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Levels" [KEYWORDS]
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively.
2. The Authentication Service Extension
1. The name of this [SMTP] service extension is "Authentication"
2. The EHLO keyword value associated with this extension is
"AUTH"
3. The AUTH EHLO keyword contains as a parameter a space
separated list of the names of available [SASL] mechanisms.
The list of available mechanisms MAY change after a successful
STARTTLS command [SMTP-TLS].
4. A new [SMTP] verb "AUTH" is defined.
5. An optional parameter using the keyword "AUTH" is added to the
MAIL FROM command, and extends the maximum line length of the
MAIL FROM command by 500 characters.
6. This extension is appropriate for the submission protocol
[SUBMIT].
3. The AUTH Command
AUTH mechanism [initial-response]
Arguments:
mechanism: A string identifying a [SASL] authentication
mechanism.
initial-response: An optional initial client response. If
present, this response MUST be [BASE64] encoded.
Restrictions:
After an AUTH command has been successfully completed, no more
AUTH commands may be issued in the same session. After a
successful AUTH command completes, a server MUST reject any
further AUTH commands with a 503 reply.
The AUTH command is not permitted during a mail transaction.
Discussion:
The AUTH command initiates a [SASL] authentication exchange
between the client and the server. The client identifies the
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SASL mechanism to use with the first parameter of the AUTH
command. If the server supports the requested authentication
mechanism, it performs the SASL exchange to authenticate the
user. Optionally, it also negotiates a security layer for
subsequent protocol interactions during this session. If the
requested authentication mechanism is not supported, the
server rejects the AUTH command with a 504 reply.
The authentication protocol exchange consists of a series of
server challenges and client responses that are specific to
the chosen [SASL] mechanism.
A server challenge is sent as a 334 reply with the text part
containing the [BASE64] encoded string supplied by the SASL
mechanism. This challenge MUST NOT contain any text other
than the BASE64 encoded challenge.
A client response consists of a line containing a [BASE64]
encoded string. If the client wishes to cancel the
authentication exchange, it issues a line with a single "*".
If the server receives such a response, it MUST reject the
AUTH command by sending a 501 reply.
The optional initial response argument to the AUTH command is
used to save a round trip when using authentication mechanisms
that support an initial client response. If the initial
response argument is omitted and the chosen mechanism requires
an initial client response, the server MUST proceed as defined
in section 5.1 of [SASL]. In SMTP, a server challenge that
contains no data is defined as a 334 reply with no text part.
Note that there is still a space following the reply code, so
the full response line is "334 ".
If the client needs to send a zero length initial response,
the client MUST transmit the response as a single equals sign
("="). This indicates that the response is present, but
contains no data.
If the client uses an initial-response argument to the AUTH
command with a SASL mechanism that does not support an initial
client send, the server MUST reject the AUTH command with a
535 reply.
If the server cannot [BASE64] decode any client response, it
MUST reject the AUTH command with a 501 reply. If the client
cannot BASE64 decode any of the server's challenges, it MUST
cancel the authentication using the "*" response. In
particular, servers and clients MUST reject (and not ignore)
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any character not explicitly allowed by the BASE64 alphabet,
and MUST reject any sequence of BASE64 characters that
contains the pad character ('=') anywhere other than the end
of the string (e.g. "=AAA" and "AAA=BBB" are not allowed).
Note that these [BASE64] strings may be of arbitrarily length.
Clients and servers MUST be able to handle the maximum encoded
size of challenges and responses generated by their supported
authentication mechanisms. This requirement is independent of
any line length limitations the client or server may have in
other parts of its protocol implementation.
If the server is unable to authenticate the client, it SHOULD
reject the AUTH command with a 535 reply unless a more
specific error code, such as one listed in Section 5, is
appropriate. Should the client successfully complete the
exchange, the SMTP server issues a 235 reply.
If a security layer is negotiated during the SASL exchange, it
takes effect for the client on the octet immediately following
the CRLF that concludes the last response generated by the
client. For the server, it takes effect immediately following
the CRLF of its success reply.
When a security layer takes effect, 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 EHLO argument,
which was not obtained from the SASL negotiation itself.
Likewise, the client MUST discard any knowledge obtained from
the server, such as the list of SMTP service extensions, which
was not obtained from the SASL negotiation itself (Note that a
client MAY compare the advertised SASL mechanisms before and
after authentication in order to detect an active down-
negotiation attack).
The client SHOULD send an EHLO command as the first command
after a successful SASL negotiation which results in the
enabling of a security layer.
When both [TLS] and SASL security layers are in effect, the
TLS encoding MUST be applied after the SASL encoding,
regardless of the order in which the layers were negotiated.
The service name specified by this protocol's profile of SASL
is "smtp".
If an AUTH command fails, the client may try another
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authentication mechanism or present different credentials by
issuing another AUTH command. Likewise, the server MUST
behave as if the client had not issued the AUTH command.
To ensure interoperability, client and server implementations
of this extension MUST implement the STARTTLS Extension [SMTP-
TLS], and the [PLAIN] SASL mechanism. Implementations MUST
support a configuration where SASL mechanisms that are
vulnerable to passive eavesdropping attacks are not advertised
or used without the presence of an external security layer
such as [TLS].
3.1. Examples
Here is an example of a client attempting AUTH PLAIN under TLS and
making use of the initial client response:
S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH GSSAPI
S: 250-STARTTLS
C: STARTTLS
S: 220 Ready to start TLS
... TLS negotiation proceeds, further commands protected by TLS layer ...
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH KERBEROS_V4 GSSAPI PLAIN
C: AUTH PLAIN dGVzdAB0ZXN0ADEyMzQ=
S: 235 Authentication successful
Here is another client that is attempting AUTH PLAIN under a TLS
layer, this time without the initial response. Parts of the
negotiation before the TLS layer was established have been omitted:
... TLS negotiation proceeds, further commands protected by TLS layer ...
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH KERBEROS_V4 GSSAPI PLAIN
C: AUTH PLAIN
(note that there is a single space following the 334 on the following line)
S: 334
C: dGVzdAB0ZXN0ADEyMzQ=
S: 235 Authentication successful
Here is an example using a mechanism which does not support an
initial client send, and includes server challenges:
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S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH KERBEROS_V4
S: 250-STARTTLS
C: AUTH KERBEROS_V4
S: 334 asYNKQ==
(the following lines are broken for editorial clarity only)
C: BAYFQU5EUkVXLkNNVS5FRFUAOCCNPBDBLMcPZH4tgQ5hEI4XrsVr0xzDubFTzrKypQ
oxLifPkfRootCpSkFvF+4K3DSnAaok9mQLYW+3MqyH5XmxyKraLHYKtyeRXX0u/X1V
O1eJBGBZLmVE
S: 334 /HEaZgpK4VA=
C: 8YDkh5Lu3cB09Pso3dIRmg==
S: 235 Authentication successful
... a security layer has been established, so the client re-issues
the EHLO command ...
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH KERBEROS_V4
S: 250-STARTTLS
4. The AUTH Parameter to the MAIL FROM command
AUTH=addr-spec
Arguments:
An addr-spec (see section 3.4.1 or [RFC2822]) that names the
identity which submitted the message to the delivery system, or
the two character sequence "<>" indicating such an identity is
unknown or insufficiently authenticated. To comply with
restrictions imposed on ESMTP parameters, the addr-spec is
encoded inside an xtext. The syntax of an xtext is described in
Section 5 of [ESMTP-DSN].
Note:
For the purposes of this discussion, "authenticated identity"
refers to the identity (if any) derived from a previous AUTH
command, while the terms "authorized identity" and "supplied
addr-spec" refer to the sender identity that is being associated
with a particular message. Note that one authenticated identity
may be able to identify messages as being sent by any number of
authorized identities within a single session. For example,
this may be the case when an SMTP server (one authenticated
identity) is processing its queue (many messages with distinct
authorized identities).
Discussion:
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The optional AUTH parameter to the MAIL FROM command allows
cooperating agents in a trusted environment to communicate the
authorization identity associated with individual messages.
If the server trusts the authenticated identity of the client to
assert that the message was originally submitted by the supplied
addr-spec, then the server SHOULD supply the same addr-spec in
an AUTH parameter when relaying the message to any other server
which supports the AUTH extension.
For this reason, servers that advertise support for this
extension MUST support the AUTH parameter to the MAIL FROM
command even when the client has not authenticated itself to the
server.
A MAIL FROM parameter of AUTH=<> indicates that the original
submitter of the message is not known. The server MUST NOT
treat the message as having been originally submitted by the
current authenticated identity.
If the AUTH parameter to the MAIL FROM command is not supplied,
the client has authenticated, and the server believes the
message is an original submission, the server MAY supply the
client's identity in the addr-spec in an AUTH parameter when
relaying the message to any server which supports the AUTH
extension.
If the server does not sufficiently trust the authenticated
identity of the client, or if the client is not authenticated,
then the server MUST behave as if the AUTH=<> parameter was
supplied. The server MAY, however, write the value of any
supplied AUTH parameter to a log file.
If an AUTH=<> parameter was supplied, either explicitly or due
to the requirement in the previous paragraph, then the server
MUST supply the AUTH=<> parameter when relaying the message to
any server which it has authenticated to using the AUTH
extension.
A server MAY treat expansion of a mailing list as a new
submission, setting the AUTH parameter to the mailing list
address or mailing list administration address when relaying the
message to list subscribers.
Note that an implementation which is hard-coded to treat all
clients as being insufficiently trusted is compliant with this
specification. In that case, the implementation does nothing
more than parse and discard syntactically valid AUTH parameters
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to the MAIL FROM command, and supply AUTH=<> parameters to any
servers which it authenticates to.
4.1. Example
An example where the original identity of the sender is trusted and
known:
C: MAIL FROM:<e=mc2@example.com> AUTH=e+3Dmc2@example.com
S: 250 OK
One example where the identity of the sender is not trusted or is
otherwise being suppressed by the client:
C: MAIL FROM:<john+@example.org> AUTH=<>
S: 250 OK
5. Error Codes
The following error codes may be used to indicate various failure
conditions:
432 A password transition is needed
This response to the AUTH command indicates that the user needs to
transition to the selected authentication mechanism. This is
typically done by authenticating once using the [PLAIN]
authentication mechanism. The selected mechanism SHOULD then work
for authentications in subsequent sessions.
534 Authentication mechanism is too weak
This response to the AUTH command indicates that the selected
authentication mechanism is weaker than server policy permits for
that user.
538 Encryption required for requested authentication mechanism
This response to the AUTH command indicates that the selected
authentication mechanism may only be used when the underlying SMTP
connection is encrypted. Note that this response code is documented
here for historical purposes only. Modern implementations SHOULD
NOT advertise mechanisms that are not permitted due to lack of
encryption, unless an encryption layer of sufficient strength is
currently being employed.
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454 Temporary authentication failure
This response to the AUTH command indicates that the authentication
failed due to a temporary server failure.
530 Authentication required
This response may be returned by any command other than AUTH, EHLO,
HELO, NOOP, RSET, or QUIT. It indicates that server policy requires
authentication in order to perform the requested action.
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form notation as specified in [ABNF].
Except as noted otherwise, all alphabetic characters are case-
insensitive. The use of upper or lower case characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.
UPALPHA = %x41-5A ;; Uppercase: A-Z
LOALPHA = %x61-7A ;; Lowercase: a-z
ALPHA = UPALPHA / LOALPHA ;; case insensitive
DIGIT = %x30-39 ;; Digits 0-9
HEXDIGIT = %x41-46 / DIGIT ;; hexidecimal digit (uppercase)
hexchar = "+" HEXDIGIT HEXDIGIT
xchar = %x21-2A / %x2C-3C / %x3E-7E
;; US-ASCII except for "+", "=", SPACE and CTL
xtext = *(xchar / hexchar)
AUTH_CHAR = ALPHA / DIGIT / "-" / "_"
auth_type = 1*20AUTH_CHAR
auth_command = "AUTH" SPACE auth_type [SPACE (base64 / "=")]
*(CRLF [base64]) CRLF
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auth_param = "AUTH=" xtext
;; The decoded form of the xtext MUST be either
;; an addr-spec or the two characters "<>"
base64 = base64_terminal /
( 1*(4base64_CHAR) [base64_terminal] )
base64_char = UPALPHA / LOALPHA / DIGIT / "+" / "/"
;; Case-sensitive
base64_terminal = (2base64_char "==") / (3base64_char "=")
continue_req = "334" SPACE [base64] CRLF
CR = %x0C ;; ASCII CR, carriage return
CRLF = CR LF
CTL = %x00-1F / %x7F ;; any ASCII control character and DEL
LF = %x0A ;; ASCII LF, line feed
SPACE = %x20 ;; ASCII SP, space
7. Security Considerations
Security issues are discussed throughout this memo.
If a client uses this extension to get an encrypted tunnel through
an insecure network to a cooperating server, it needs to be
configured to never send mail to that server when the connection is
not mutually authenticated and encrypted. Otherwise, an attacker
could steal the client's mail by hijacking the [SMTP] connection and
either pretending the server does not support the Authentication
extension or causing all AUTH commands to fail.
Before the [SASL] negotiation 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 SASL negotiation upon
the establishment of a security layer.
This mechanism does not protect the TCP port, so an active attacker
may redirect a relay connection attempt (i.e. a connection between
two MTAs) to the submission port [SUBMIT]. The AUTH=<> parameter
prevents such an attack from causing a relayed message, in the
absence of other envelope authentication, from picking up the
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authentication of the relay client.
A message submission client may require the user to authenticate
whenever a suitable [SASL] mechanism is advertised. Therefore, it
may not be desirable for a submission server [SUBMIT] to advertise a
SASL mechanism when use of that mechanism grants the clients no
benefits over anonymous submission.
This extension is not intended to replace or be used instead of end-
to-end message signature and encryption systems such as [S/MIME] or
[PGP]. This extension addresses a different problem than end-to-end
systems; it has the following key differences:
1. It is generally useful only within a trusted enclave.
2. It protects the entire envelope of a message, not just the
message's body.
3. It authenticates the message submission, not authorship of the
message content.
4. When mutual authentication is used along with a security
layer, it can give the sender some assurance that the message
was successfully delivered to the next hop.
Additional security considerations are mentioned in the [SASL]
specification.
8. IANA Considerations
This document requests that the IANA update the entry for the "smtp"
SASL protocol name to point at this document.
This document requests that the IANA register the Authentication
SMTP service extension as defined in Section 2 of this document.
9. Intellectual Property Rights
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
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claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification
can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
10. Copyright
Copyright (C) The Internet Society (2003). 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.
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.
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11. References
The following documents contain normative definitions or
specifications that are necessary for correct understanding of this
protocol:
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[ESMTP-DSN] Moore, K., "SMTP Service Extension for Delivery Status
Notifications", RFC 1891, January 1996.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[PLAIN] Newman, C. "Using TLS with IMAP, POP3, and ACAP", RFC 2595,
June 1999.
[RFC2822] Resnick, P. "Internet Message Format", RFC 2822, April 2001.
[SASL] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[SMTP-TLS] Hoffman, P. "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[SUBMIT] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
December 1998.
The following references are for informational purposes only:
[PGP] Elkins, M., "MIME Security with Pretty Good Privacy (PGP)",
RFC 2015, October 1996.
[S/MIME] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1999.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
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12. Changes Since RFC 2554
1. Clarify that servers MUST support the use of the
AUTH=addr-spec parameter to MAIL FROM, even when the
client is not authenticated.
2. Clarify the initial-client-send requirements, and give
additional examples.
3. Update references to newer versions of various
specifications.
4. Require the minimum implementation of TLS+PLAIN.
5. Clarify that the mechanism list can change.
6. Deprecate the use of the 538 response code.
7. General other editorial clarifications.
13. Author's Address:
Robert Siemborski
Carnegie Mellon, Andrew Systems Group
Cyert Hall 207
5000 Forbes Avenue
Pittsburgh, PA 15213
+1 412 268 7456
rjs3+@andrew.cmu.edu
14. Acknowledgments:
The author would like to acknowledge the contributions of John Myers
and other contributors to RFC 2554, on which this document draws
from heavily.
The author would also like to thank Ken Murchison and Mark Crispin
for the time they devoted to reviewing early drafts of this
document.
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