Network Working Group Robert Siemborski
INTERNET-DRAFT Carnegie Mellon University
Intended Category: Proposed Standard April, 2004
POP3 SASL Authentication Mechanism
<draft-siemborski-rfc1734bis-03.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 profile of the Simple Authentication and
Security Layer (SASL) for the Post Office Protocol (POP3). This
extension allows a POP3 client to 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.
In order to consolidate all of the authentication related
information for POP3 into a single document, this document obsoletes
RFC 1734 and RFC 3206, replacing them as a Proposed Standard. It
also updates information contained in Section 6.3 and Section 8 of
RFC 2449.
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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 Levels" [KEYWORDS]
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
2. Introduction
The [POP3] AUTH command [POP3-AUTH] in has suffered several problems
in its specification. The first is that it was very similar to a
[SASL] framework, but pre-dated the initial SASL specification. It
was therefore missing some key components, such as a way to list the
available authentication mechanisms.
Later, [POP3-EXT] attempted to remedy this situation by adding the
CAPA command and allowing an initial client response to the AUTH
command, however problems in the clarity of the specification of how
the initial client response was to be handled remained.
Additionally, there is yet another document, [POP3-CODES], that
provides additional response codes that are useful during
authentication. Together, this means creating a full POP3 AUTH
implementaiton requires an understanding of material in atleast six
different documents.
This document attempts to combine all of the POP3 SASL
authentication related details into a single document, in addition
to clarifying and updating the older specifications where
appropriate.
3. The SASL Capability
This section supercedes the definition of the SASL Capability in
section 6.3 of [POP3-EXT].
CAPA tag:
SASL
Arguments:
Supported SASL Mechanisms
Standard Commands Affected
None
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Announced states / possible differences:
both / no
Commands valid in states:
AUTHORIZATION
Specification Reference:
This Document, [SASL]
Discussion
The SASL capability permits the use of the AUTH command (as
defined in section 4 of this document) to begin a [SASL]
negotiation. The arguments to the SASL capability is a space-
separated list of SASL mechanisms which are supported.
If a server either does not support the CAPA command or does not
advertise the SASL capability, clients SHOULD NOT attempt the
AUTH command. If a client does attempt the AUTH command in such
a situation, it MUST NOT supply the client initial response
parameter (for backwards compatibility with [POP3-AUTH]).
Note that the list of available mechanisms MAY change after a
successful STLS command [POP3-TLS]. Additionally,
implementations MAY choose to omit the SASL capability after a
successful AUTH command has been completed.
Example
S: +OK pop.example.com BlurdyBlurp POP3 server ready
C: CAPA
S: +OK List of capabilities follows
S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
S: STLS
S: IMPLEMENTATION BlurdyBlurp POP3 server
S: .
4. 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 encoded as specified in Section
3 of [BASE64].
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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 -ERR reply.
The AUTH command may only be given during the AUTHORIZATION
state.
Discussion:
The AUTH command initiates a [SASL] authentication exchange
between the client and the server. The client identifies the
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 -ERR 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 line consisting of a "+"
character followed by a single space and a string encoded as
specified in Section 3 of [BASE64]. This challenge MUST NOT
contain any text other than the BASE64 encoded challenge.
A client response consists of a line containing a string
encoded as defined in Section 3 of [BASE64]. 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 -ERR 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 POP3, a server challenge with no
data is defined as line with only a "+" followed by a single
space. It MUST NOT contain any other data.
For the purposes of the initial client response, the line
length limitation defined in [POP3-EXT] still applies. If a
client initial send would cause the AUTH command to exceed
this length, the client MUST NOT use the initial response
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parameter (and instead proceed as defined in section 5.1 of
[SASL]).
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
-ERR reply.
If the server cannot [BASE64] decode any client response, it
MUST reject the AUTH command with a -ERR 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)
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 (excepting the initial client
response) 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 MUST
reject the AUTH command with a -ERR reply. Should the client
successfully complete the exchange, the server issues a +OK
reply. Additionally, upon success, the POP3 session enters
the TRANSACTION state.
The authorization identity generated by this [SASL] exchange
is a simple username, and MUST use the [SASLprep] profile of
the [StringPrep] algorithm to prepare these names for
matching. If preparation of the authorization identity fails
or results in an empty string (unless it was transmitted as
the empty string), the server MUST fail the authentication.
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
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the CRLF of its success reply.
When a security layer takes effect, the server MUST discard
any knowledge previously obtained from the client, 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 available POP3 service extensions. After
a security layer is established, the server SHOULD NOT
advertise either the SASL or the STLS extension.
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 "pop".
If an AUTH command fails, the client may try another
authentication mechanism or present different credentials by
issuing another AUTH command (or by using one of the other
[POP3] authentication mechanisms). 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 [DIGEST-MD5] SASL
mechanism.
4.1. 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
AUTH_CHAR = ALPHA / DIGIT / "-" / "_"
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auth_type = 1*20AUTH_CHAR
auth_command = "AUTH" SPACE auth_type [SPACE (base64 / "=")]
*(CRLF [base64]) CRLF
base64 = base64_terminal /
( 1*(4base64_CHAR) [base64_terminal] )
base64_char = UPALPHA / LOALPHA / DIGIT / "+" / "/"
;; Case-sensitive
base64_terminal = (2base64_char "==") / (3base64_char "=")
continue_req = "+" SPACE [base64] CRLF
CR = %x0C ;; ASCII CR, carriage return
CRLF = CR LF
LF = %x0A ;; ASCII LF, line feed
SPACE = %x20 ;; ASCII SP, space
4.2. Examples
Here is an example of a client attempting AUTH PLAIN under TLS and
making use of the initial client response:
S: +OK pop.example.com BlurdyBlurp POP3 server ready
C: CAPA
S: +OK List of capabilities follows
S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
S: STLS
S: IMPLEMENTATION BlurdyBlurp POP3 server
S: .
C: STLS
S: +OK Begin TLS negotiation now
... TLS negotiation proceeds, further commands protected by TLS layer ...
C: CAPA
S: +OK List of capabilities follows
S: SASL PLAIN KERBEROS_V4 GSSAPI ANONYMOUS
S: IMPLEMENTATION BlurdyBlurp POP3 server
S: .
C: AUTH PLAIN dGVzdAB0ZXN0AHRlc3Q=
S: +OK Maildrop locked and ready
Here is another client that is attempting AUTH PLAIN under a TLS
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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: CAPA
S: +OK List of capabilities follows
S: SASL PLAIN KERBEROS_V4 GSSAPI ANONYMOUS
S: IMPLEMENTATION BlurdyBlurp POP3 server
S: .
C: AUTH PLAIN
(note that there is a space following the '+' on the following line)
S: +
C: dGVzdAB0ZXN0AHRlc3Q=
S: +OK Maildrop locked and ready
Here is an example using a mechanism which does not support an
initial client send, and includes server challenges:
S: +OK pop.example.com BlurdyBlurp POP3 server ready
C: CAPA
S: +OK List of capabilities follows
S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
S: STLS
S: IMPLEMENTATION BlurdyBlurp POP3 server
S: .
C: AUTH KERBEROS_V4
S: + ezLUFA==
(the following lines are broken for editorial clarity only)
C: BAYFQU5EUkVXLkNNVS5FRFUAOCCXeMiVyFe9K6Nwne7+sPLgIoF9YQ5ePfxUsMlJAf
C7aoNySU8nrqS9m8JAddsUeuyc5HFXXovaKLrZNo2bTLH0Lyolwy0W9ryJDojbKmHy
zSMqFsGD4EL0
S: + Z74fTwDw7KQ=
C: vSAF7ha6qotK2UHUgKlsEA==
S: +OK Maildrop locked and ready
... at this point a security layer has been established and additional
commands and responses proceed within it ...
5. Extended POP3 Response Codes
This section defines four POP3 response codes which can be used to
determine the reason for a failed login (provided that the server
advertises the RESP-CODES capability [POP3-EXT]). These definitions
supercede those in [POP3-EXT] and [POP3-CODES].
It is RECOMMENDED that server applications use these codes when
possible to allow clients a straightforward, interoperable way to
determine the cause of an authentication failure (as opposed to
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parsing error text).
5.1. The LOGIN-DELAY Response Code
This occurs on an -ERR response to an AUTH, USER (see note), PASS or
APOP command and indicates that the user has logged in recently and
will not be allowed to login again until the login delay period has
expired.
Please see the Security Considerations section of this document for
an important note about returning this code in response to the USER
command.
5.2. The IN-USE Response Code
This occurs on an -ERR response to an AUTH, APOP, or PASS command.
It indicates the authentication was successful, but the user's
maildrop is currently in use (probably by another POP3 client).
5.3. The AUTH Response Code
The AUTH response code informs the client that there is a problem
with the user's credentials. This might be an incorrect password,
an unknown user name, an expired account, an attempt to authenticate
in violation of policy (such as from an invalid location or during
an unauthorized time), or some other problem.
The AUTH response code is valid with an -ERR response to any
authentication command including AUTH, USER (see the note in the
Security Considerations section of this document), PASS, or APOP.
Servers which include the AUTH response code with any authentication
failure SHOULD support the CAPA command [POP3-EXT] and SHOULD
include the AUTH-RESP-CODE capability (defined in the next section)
in the CAPA response. AUTH-RESP-CODE assures the client that only
errors with the AUTH code are caused by credential problems.
5.3.1. The AUTH-RESP-CODE Capability
CAPA tag:
AUTH-RESP-CODE
Arguments:
none
Added commands:
none
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Standard commands affected:
none
Announced states / possible differences:
both / no
Commands valid in states:
n/a
Specification reference:
this document
Discussion:
The AUTH-RESP-CODE capability indicates that the server includes
the AUTH response code with any authentication error caused by a
problem with the user's credentials.
5.4. The SYS Response Code
The SYS response code announces that a failure is due to a system
error, as opposed to the user's credentials or an external
condition. It is hierarchical, with two possible second-level
codes: TEMP and PERM. (Case is not significant at any level of the
hierarchy.)
SYS/TEMP indicates a problem which is likely to be temporary in
nature, and therefore there is no need to alarm the user, unless the
failure persists. Examples might include a central resource which
is currently locked or otherwise temporarily unavailable,
insufficient free disk or memory, etc.
SYS/PERM is used for problems which are unlikely to be resolved
without intervention. It is appropriate to alert the user and
suggest that the organization's support or assistance personnel be
contacted. Examples include corrupted mailboxes, system
configuration errors, etc.
The SYS response code is valid with an -ERR response to any command.
6. Security Considerations
Security issues are discussed throughout this memo.
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.
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Servers MAY implement a policy whereby the connection is dropped
after a number of failed authentication attempts. If they do so,
they SHOULD NOT drop the connection until atleast 3 attempts to
authenticate have failed.
Implementations MUST support a configuration where [SASL] mechanisms
that are vulnerable to passive eavesdropping attacks (such as
[PLAIN]) are not advertised or used without the presence of an
external security layer such as [TLS].
Returning the LOGIN-DELAY or AUTH response codes to the USER command
avoids the work of authenticating the user but is likely to reveal
information to the client about the existence of the account in
question. Unless the server is operating in an environment where
user names are not secret (for example, many popular email clients
advertise the POP server and user name in an outgoing mail header),
or where server access is restricted, or the server can verify that
the connection is to the same user, the the server SHOULD NOT issue
this response code to the USER command. The server still saves the
cost of opening the maildrop, which in some environments is the most
expensive step.
7. IANA Considerations
This document requests that the IANA update the entry for the "pop"
SASL protocol name to point at this document.
This document requests that the IANA update the entry for the SASL
POP3 capability to be as defined in Section 3 of this document.
This document requests that the IANA update the entry for the LOGIN-
DELAY, IN-USE, AUTH, SYS/TEMP, and SYS/PERM POP3 response codes to
this document.
This document requests that the IANA update the entry for the AUTH-
RESP-CODE capability to be as defined in Section 5.3.1 of this
document.
8. Protocol Actions
[RFC Editor: Remove this section before publication]
This document obsoletes RFC 1734 and replaces it as a Proposed
Standard. By moving RFC 1734 to Historic, RFC 1731 can also be
moved to Historic (as RFC 1734 was the last document to have a
normative reference).
This document obsoletes RFC 3206 and replaces it as a Proposed
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Standard.
It also updates information contained in Section 6.3 and Section 8
of RFC 2449.
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
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
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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:
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[DIGEST-MD5]
Leach, P., Melnikov, A., and Newman C., "Using Digest
Authentication as a SASL Mechanism", draft-ietf-sasl-
rfc2831bis-*.txt, a work in progress.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[POP3] Myers, J. and Rose, M., "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[POP3-EXT] Gellens, R., Newman, C., and Lundblade, L., "POP3 Extension
Mechanism", RFC 2449, November 1998.
[POP3-TLS] Newman, C., "Using TLS with IMAP, POP3, and ACAP", RFC 2595,
June 1999.
[SASL] Melnikov, A., "Simple Authentication and Security Layer
(SASL)", draft-ietf-sasl-rfc2222bis-*.txt, a work in
progress.
[SASLprep] Zeilega, K., "SASLprep: Stringprep profile for user names
and passwords", draft-ietf-sasl-saslprep-*.txt, a work in
progress
[StringPrep]
Hoffman, P. and Blanchet, M., "Preparation of
Internationalized Strings ("stringprep")", draft-hoffman-
rfc3454bis-*.txt, a work in progress
The following references are for informational purposes only:
[PLAIN] Zeilenga, K., "The Plain SASL Mechanism", draft-ietf-sasl-
plain-*.txt, a work in progress.
[POP3-AUTH] Myers, J., "POP3 AUTHentication Command", RFC 1734, January
1994.
[POP3-CODES]
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Gellens, R., "The SYS and AUTH POP Response Codes", RFC
3206, February 2002.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
12. Changes From RFC 1734, RFC 2449, and RFC 3206
1. The SASL-based semantics defined in RFC 2449 are now
normative for the AUTH extension.
2. Clarifications and examples of the proper behavior of
initial client response handling.
3. Minimum requirement of support for DIGEST-MD5.
4. Clarify ordering of TLS and SASL security layers.
5. Update references to newer versions of various
specifications.
6. Clarify that the mechanism list can change.
7. Add the use of the SASLprep profile for preparing
authorization identities.
8. General other editorial clarifications.
9. Consolidation of all applicable information into a
single document.
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, Randall Gellens, Chris Newman, Laurence Lundblade, and other
contributors to RFC 1734, RFC 2554, and RFC 3206, on which this
document draws heavily.
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The author would also like to thank Ken Murchison, Alexey Melnikov,
and Mark Crispin for the time they devoted to reviewing early drafts
of this document.
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Table of Contents
1. How to Read This Document . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The SASL Capability . . . . . . . . . . . . . . . . . . . . . . . 3
4. The AUTH Command . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Extended POP3 Response Codes . . . . . . . . . . . . . . . . . . 9
5.1. The LOGIN-DELAY Response Code . . . . . . . . . . . . . . . . . 10
5.2. The IN-USE Response Code . . . . . . . . . . . . . . . . . . . 10
5.3. The AUTH Response Code . . . . . . . . . . . . . . . . . . . . 10
5.3.1. The AUTH-RESP-CODE Capability . . . . . . . . . . . . . . . . 10
5.4. The SYS Response Code . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 12
8. Protocol Actions . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Intellectual Property Rights . . . . . . . . . . . . . . . . . . 13
10. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
12. Changes From RFC 1734, RFC 2449, and RFC 3206 . . . . . . . . . 16
13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 16
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