RFC5034
Network Working Group R. Siemborski Request for Comments: 5034 Google, Inc. Obsoletes: 1734 A. Menon-Sen Updates: 2449 Oryx Mail Systems GmbH Category: Standards Track July 2007
The Post Office Protocol (POP3)
Simple Authentication and Security Layer (SASL) Authentication Mechanism
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 IETF Trust (2007).
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.
This document seeks to consolidate the information related to POP3 AUTH into a single document. To this end, this document obsoletes and replaces RFC 1734, and updates the information contained in Section 6.3 of RFC 2449.
Contents
Introduction
The POP3 (see RFC1939) AUTH command (see RFC1734) has suffered several problems in its specification. The first is that it was very similar to a SASL framework defined by RFC4422, 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, RFC2449 attempted to remedy this situation by adding the CAPA command and allowing an initial client response with the AUTH command, but problems remained in the clarity of the specification of how the initial client response was to be handled.
Together, this means creating a full POP3 AUTH implementation requires an understanding of material in at least five different documents (and RFC3206 provides additional response codes that are useful during authentication).
This document attempts to combine the information in RFC1734 and RFC2449 to simplify this situation. Additionally, it aims to clarify and update the older specifications where appropriate.
Conventions Used in This Document
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.
In examples, "C:" and "S:" indicate lines sent by the client and server respectively.
Formal syntax is defined by RFC4234.
The SASL Capability
This section supersedes the definition of the SASL Capability in section 6.3 of RFC2449.
CAPA tag:
SASL
Arguments:
Supported SASL Mechanisms
Added commands:
AUTH
Standard commands affected:
None
Announced states / possible differences:
both / no
Commands valid in states:
AUTHORIZATION
Specification reference:
This document and RFC4422
Discussion:
The SASL capability permits the use of the AUTH command (as defined in Section 4 of this document) to begin a SASL negotiation (as defined in RFC4422). The argument to the SASL capability is a space-separated list of SASL mechanisms that 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 RFC1734).
Note that the list of available mechanisms MAY change after a successful STLS command (see RFC2595). However, as required by RFC2449, implementations MUST continue to include the SASL capability even after a successful AUTH command has been completed (even though no further AUTH commands may be issued).
Example
S: +OK pop.example.com BlurdyBlurp POP3 server ready C: CAPA S: +OK List of capabilities follows S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS S: STLS S: IMPLEMENTATION BlurdyBlurp POP3 server S: .
The AUTH Command
AUTH mechanism [initial-response]
Arguments:
mechanism: A string identifying a SASL authentication
mechanism.
initial-response: An optional initial client response, as
defined in Section 3 of RFC4422. If present, this response
MUST be encoded as Base64 (specified in Section 4 of
RFC4648), or consist only of the single character "=", which
represents an empty initial response.
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 an -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 an -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
using Base64, as specified in Section 4 of RFC4648. This
line MUST NOT contain any text other than the BASE64-encoded
challenge.
A client response consists of a line containing a string
encoded as 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 an -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 by issuing
an empty challenge, as defined in Section 3 of RFC4422. In
POP3, an empty server challenge is defined as a 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 255-octet
limit on the length of a single command, defined in Section 4
of RFC2449, still applies. If specifying an initial response
would cause the AUTH command to exceed this length, the client
MUST NOT use the initial-response parameter (and must proceed
instead by sending its initial response after an empty
challenge from the server, as in Section 3 of RFC4422).
If the client needs to send a zero-length initial response, it
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 an
-ERR reply.
If the server cannot Base64 decode a client response, it MUST
reject the AUTH command with an -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).
Excepting the initial client response, these BASE64 strings may
be of arbitrary length, depending on the authentication
mechanism in use. Clients and servers MUST be able to handle
the largest encoded challenges and responses generated by the
authentication mechanisms they support. 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 an -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 the SASL exchange is a
simple username, and SHOULD use the SASLprep profile (see
RFC4013) of the StringPrep algorithm (see RFC3454) 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
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.
When both Transport Layer Security (TLS) (see RFC4346) and SASL security layers are in effect, the TLS encoding MUST be applied after the SASL encoding when sending data. (According to RFC2595, STLS can only be issued before AUTH in any case.)
Note that POP3 does not allow for additional data to be sent
with a message indicating a successful outcome (see Section 3.6
of RFC4422).
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 PLAIN SASL mechanism
RFC4616 running over TLS RFC2595.
A server implementation MUST implement a configuration in which
it does NOT advertise or permit any plaintext password
mechanisms, unless the STLS command has been used to negotiate
a TLS session (see RFC2595). As described by RFC 4616, this
configuration SHOULD be the default configuration. Before
using a plaintext password mechanism over a TLS session, client
implementations MUST verify the TLS server certificate as
required by RFC 2595, Section 2.4. Client and server
implementations SHOULD implement additional SASL mechanisms
that do not send plaintext passwords, such as the GSSAPI
RFC4752 mechanism.
Formal Syntax
The following syntax specification uses the Augmented Backus-Naur Form notation as specified in RFC4234. The rules CRLF, ALPHA, and DIGIT are imported from RFC4234. The sasl-mech rule is from RFC4422.
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.
auth-command = "AUTH" SP sasl-mech [SP initial-response]
*(CRLF [base64]) [CRLF cancel-response] CRLF
initial-response = base64 / "="
cancel-response = "*"
base64 = base64-terminal /
( 1*(4base64-CHAR) [base64-terminal] )
base64-char = ALPHA / DIGIT / "+" / "/"
;; Case-sensitive
base64-terminal = (2base64-char "==") / (3base64-char "=")
continue-req = "+" SP [base64] CRLF
Additionally, the ABNF specified in RFC2449 is updated as follows:
response =/ continue-req
Examples
Here is an example of a client attempting AUTH PLAIN (see RFC4616) 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 DIGEST-MD5 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 DIGEST-MD5 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 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 DIGEST-MD5 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 in which the exchange begins with a server challenge (the long lines are broken for editorial clarity only):
S: +OK pop.example.com BlurdyBlurp POP3 server ready
C: CAPA
S: +OK List of capabilities follows
S: SASL DIGEST-MD5 GSSAPI ANONYMOUS
S: STLS
S: IMPLEMENTATION BlurdyBlurp POP3 server
S: .
C: AUTH DIGEST-MD5
S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0
RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh
cnNldD11dGYtOA==
C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2
QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw
MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9In
BvcC9lbHdvb2QuaW5ub3NvZnQuY29tIixyZXNwb25zZT1iMGQ1NmQyZjA1
NGMyNGI2MjA3MjMyMjEwNjQ2OGRiOSxxb3A9YXV0aA==
S: + cnNwYXV0aD0wYjk3MTQ2MmNlZjVlOGY5MzBkYjlhMzNiMDJmYzlhMA==
C:
S: +OK Maildrop locked and ready
Security Considerations
Security issues are discussed throughout this document.
IANA Considerations
The IANA has updated its site to refer to this RFC instead of RFC1734 in http://www.iana.org/assignments/pop3-extension-mechanism (the POP3 extension registry), and also in http://www.iana.org/assignments/gssapi-service-names (the GSSAPI/SASL service name registry).
Acknowledgments
The authors would like to acknowledge the contributions of John Myers, Randall Gellens, Chris Newman, Laurence Lundblade, and other contributors to RFC 1734 and RFC 2554, on which this document draws heavily.
The authors would also like to thank Ken Murchison, Randall Gellens, Alexey Melnikov, Mark Crispin, Arnt Gulbrandsen, Lisa Dusseault, Frank Ellermann, and Philip Guenther for their reviews of this document.
10. Changes From RFC 1734, RFC 2449.
1. Updated references to newer versions of various specifications,
particularly RFC 4422.
2. The SASL-based semantics defined in RFC 2449 are now normative for
the AUTH extension.
3. The proper behaviour and handling of initial client responses is
defined, with examples and references to SASL.
4. New minimum requirement of support for TLS+PLAIN.
5. The SASLprep profile SHOULD be used to prepare authorization
identities.
6. Clarify that the TLS encoding should be applied after any encoding
applied by SASL security layers.
7. Note that the mechanism list can change after STLS.
8. Explicitly mention that "=" means a zero-length initial response.
9. Note that POP3 doesn't allow additional data to be sent with +OK.
11. Normative References
RFC1939 Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
RFC2449 Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
Mechanism", RFC 2449, November 1998.
RFC2595 Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
RFC3454 Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
RFC4013 Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.
RFC4234 Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 4234, October 2005.
RFC4422 Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422, June 2006.
RFC4648 Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
RFC4616 Zeilenga, K., Ed., "The PLAIN Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4616, August 2006.
12. Informative References
RFC1734 Myers, J., "POP3 AUTHentication command", RFC 1734,
December 1994.
RFC3206 Gellens, R., "The SYS and AUTH POP Response Codes", RFC
3206, February 2002.
RFC4346 Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
RFC4752 Melnikov, A., Ed., "The Kerberos V5 ("GSSAPI") Simple
Authentication and Security Layer (SASL) Mechanism", RFC
4752, November 2006.
Authors' Addresses
Robert Siemborski Google, Inc. 1600 Ampitheatre Parkway Mountain View, CA 94043
Phone: +1 650 623 6925 EMail: [email protected]
Abhijit Menon-Sen Oryx Mail Systems GmbH
EMail: [email protected]
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