7 Network Working Group Robert Siemborski
8 INTERNET-DRAFT Carnegie Mellon University
9 Intended Category: Proposed Standard October, 2003
12 POP3 SASL Authentication Mechanism
14 <draft-siemborski-rfc1734bis-00.txt>
18 This document is an Internet-Draft and is in full conformance with
19 all provisions of Section 10 of RFC2026.
21 Internet-Drafts are working documents of the Internet Task Force
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25 Internet-Drafts are draft documents valid for a maximum of six
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28 reference material or to cite them other than as "work in progress."
30 The list of current Internet-Drafts can be accessed at
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34 http://www.ietf.org/shadow.html.
36 Distribution of this memo is unlimited.
40 This document defines a profile of the Simple Authentication and
41 Security Layer (SASL) for the Post Office Protocol (POP3). This
42 extension allows a POP3 client to indicate an authentication
43 mechanism to the server, perform an authentication protocol
44 exchange, and optionally negotiate a security layer for subsequent
45 protocol interactions during this session.
47 This document obsoletes RFC 1734 and replaces it as a Proposed
48 Standard. It also updates information contained in Section 6.3 of
51 1. How to Read This Document
53 The key words "MUST, "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
54 "RECOMMENDED", and "MAY" in this document are to be interpreted as
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70 1. How to Read This Document . . . . . . . . . . . . . . . . . . . . 1
71 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
72 3. The SASL Capability . . . . . . . . . . . . . . . . . . . . . . . 3
73 4. The AUTH Command . . . . . . . . . . . . . . . . . . . . . . . . 4
74 4.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
75 5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 8
76 6. Security Considerations . . . . . . . . . . . . . . . . . . . . . 9
77 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 9
78 8. Intellectual Property Rights . . . . . . . . . . . . . . . . . . 9
79 9. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
80 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
81 11. Changes From RFC 1734 and RFC 2449 . . . . . . . . . . . . . . . 11
82 12. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
83 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 12
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124 POP3 SASL Authentication Mechanism October, 2003
127 defined in "Key words for use in RFCs to Indicate Requirement
130 In examples, "C:" and "S:" indicate lines sent by the client and
135 The AUTH command [POP3-AUTH] in [POP3] has suffered several
136 deficiencies in its specification. The first is that it was very
137 similar to a [SASL] framework, but pre-dated the initial SASL
138 specification. It was therefore missing some key components, such
139 as a way to list the available authentication mechanisms. Later,
140 [POP3-EXT] attempted to remedy this situation by adding the CAPA
141 command, and allowing an initial client response to the AUTH
142 command, however problems in the specification of how the initial
143 client response was to be handled remained. This document attempts
144 to combine all of the POP3 SASL authentication related details into
147 3. The SASL Capability
149 This section supercedes the definition of the SASL Capability in
150 section 6.3 of [POP3-EXT].
156 Supported SASL Mechanisms
158 Standard Commands Affected
161 Announced states / possible differences:
164 Commands valid in states:
167 Specification Reference:
168 This Document, [SASL]
171 The SASL capability permits the use of the AUTH command (as
172 defined in section 4 of this document) to begin a [SASL]
173 negotiation. The arguments to the SASL capability is a space-
174 separated list of SASL mechanisms which are supported.
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187 If a server either does not support the CAPA command or does not
188 advertise the SASL capability, clients SHOULD NOT attempt the
189 AUTH command. If a client does attempt the AUTH command in such
190 a situation, it MUST NOT supply the client initial response
191 parameter (for backwards compatibility with [POP3-AUTH]).
193 Note that the list of available mechanisms MAY change after a
194 successful STLS command [POP3-TLS]. Additionally,
195 implementations MAY choose to omit the SASL capability after a
196 successful AUTH command has been completed.
200 S: +OK pop.example.com BlurdyBlurp POP3 server ready
202 S: +OK List of capabilities follows
203 S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
205 S: IMPLEMENTATION BlurdyBlurp POP3 server
211 AUTH mechanism [initial-response]
214 mechanism: A string identifying a [SASL] authentication
217 initial-response: An optional initial client response. If
218 present, this response MUST be [BASE64] encoded.
221 After an AUTH command has been successfully completed, no more
222 AUTH commands may be issued in the same session. After a
223 successful AUTH command completes, a server MUST reject any
224 further AUTH commands with a -ERR reply.
226 The AUTH command may only be given during the AUTHORIZATION
230 The AUTH command initiates a [SASL] authentication exchange
231 between the client and the server. The client identifies the
232 SASL mechanism to use with the first parameter of the AUTH
233 command. If the server supports the requested authentication
234 mechanism, it performs the SASL exchange to authenticate the
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247 user. Optionally, it also negotiates a security layer for
248 subsequent protocol interactions during this session. If the
249 requested authentication mechanism is not supported, the
250 server rejects the AUTH command with a -ERR reply.
252 The authentication protocol exchange consists of a series of
253 server challenges and client responses that are specific to
254 the chosen [SASL] mechanism.
256 A server challenge is sent as a line consisting of a "+"
257 character followed by a single space and a [BASE64] encoded
258 string supplied by the SASL mechanism. This challenge MUST
259 NOT contain any text other than the BASE64 encoded challenge.
261 A client response consists of a line containing a [BASE64]
262 encoded string. If the client wishes to cancel the
263 authentication exchange, it issues a line with a single "*".
264 If the server receives such a response, it MUST reject the
265 AUTH command by sending a -ERR reply.
267 The optional initial response argument to the AUTH command is
268 used to save a round trip when using authentication mechanisms
269 that support an initial client response. If the initial
270 response argument is omitted and the chosen mechanism requires
271 an initial client response, the server MUST proceed as defined
272 in section 5.1 of [SASL]. In POP3, a server challenge with no
273 data is defined as line with only a "+" followed by a single
274 space. It MUST NOT contain any other data.
276 For the purposes of the initial client response, the line
277 length limitation defined in [POP3-EXT] still applies. If a
278 client initial send would cause the AUTH command to exceed
279 this length, the client MUST NOT use the initial response
280 parameter (and instead proceed as defined in section 5.1 of
283 If the client needs to send a zero-length initial response,
284 the client MUST transmit the response as a single equals sign
285 ("="). This indicates that the response is present, but
288 If the client uses an initial-response argument to the AUTH
289 command with a SASL mechanism that does not support an initial
290 client send, the server MUST reject the AUTH command with a
293 If the server cannot [BASE64] decode any client response, it
294 MUST reject the AUTH command with a -ERR reply. If the client
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307 cannot BASE64 decode any of the server's challenges, it MUST
308 cancel the authentication using the "*" response. In
309 particular, servers and clients MUST reject (and not ignore)
310 any character not explicitly allowed by the BASE64 alphabet,
311 and MUST reject any sequence of BASE64 characters that
312 contains the pad character ('=') anywhere other than the end
313 of the string (e.g. "=AAA" and "AAA=BBB" are not allowed).
315 Note that these [BASE64] strings (excepting the initial client
316 response) may be of arbitrarily length. Clients and servers
317 MUST be able to handle the maximum encoded size of challenges
318 and responses generated by their supported authentication
319 mechanisms. This requirement is independent of any line
320 length limitations the client or server may have in other
321 parts of its protocol implementation.
323 If the server is unable to authenticate the client, it MUST
324 reject the AUTH command with a -ERR reply. Should the client
325 successfully complete the exchange, the server issues a +OK
326 reply. Additionally, upon success, the POP3 session enters
327 the TRANSACTION state.
329 If a security layer is negotiated during the SASL exchange, it
330 takes effect for the client on the octet immediately following
331 the CRLF that concludes the last response generated by the
332 client. For the server, it takes effect immediately following
333 the CRLF of its success reply.
335 When a security layer takes effect, the server MUST discard
336 any knowledge previously obtained from the client, which was
337 not obtained from the SASL negotiation itself. Likewise, the
338 client MUST discard any knowledge obtained from the server,
339 such as the list of available POP3 service extensions. After
340 a security layer is established, the server SHOULD NOT
341 advertise either the SASL or the STLS extension.
343 When both [TLS] and SASL security layers are in effect, the
344 TLS encoding MUST be applied after the SASL encoding,
345 regardless of the order in which the layers were negotiated.
347 The service name specified by this protocol's profile of SASL
350 If an AUTH command fails, the client may try another
351 authentication mechanism or present different credentials by
352 issuing another AUTH command (or by using one of the other
353 [POP3] authentication mechanisms). Likewise, the server MUST
354 behave as if the client had not issued the AUTH command.
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367 To ensure interoperability, client and server implementations
368 of this extension MUST implement the STLS extension, and the
369 PLAIN SASL mechanism [POP3-TLS]. Implementations MUST support
370 a configuration where [SASL] mechanisms that are vulnerable to
371 passive eavesdropping attacks are not advertised or used
372 without the presence of an external security layer such as
377 Here is an example of a client attempting AUTH PLAIN under TLS and
378 making use of the initial client response:
380 S: +OK pop.example.com BlurdyBlurp POP3 server ready
382 S: +OK List of capabilities follows
383 S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
385 S: IMPLEMENTATION BlurdyBlurp POP3 server
388 S: +OK Begin TLS negotiation now
389 ... TLS negotiation proceeds, further commands protected by TLS layer ...
391 S: +OK List of capabilities follows
392 S: SASL PLAIN KERBEROS_V4 GSSAPI ANONYMOUS
393 S: IMPLEMENTATION BlurdyBlurp POP3 server
395 C: AUTH PLAIN dGVzdAB0ZXN0AHRlc3Q=
396 S: +OK Maildrop locked and ready
398 Here is another client that is attempting AUTH PLAIN under a TLS
399 layer, this time without the initial response. Parts of the
400 negotiation before the TLS layer was established have been omitted:
402 ... TLS negotiation proceeds, further commands protected by TLS layer ...
404 S: +OK List of capabilities follows
405 S: SASL PLAIN KERBEROS_V4 GSSAPI ANONYMOUS
406 S: IMPLEMENTATION BlurdyBlurp POP3 server
409 (note that there is a space following the '+' on the following line)
411 C: dGVzdAB0ZXN0AHRlc3Q=
412 S: +OK Maildrop locked and ready
414 Here is an example using a mechanism which does not support an
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427 initial client send, and includes server challenges:
429 S: +OK pop.example.com BlurdyBlurp POP3 server ready
431 S: +OK List of capabilities follows
432 S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
434 S: IMPLEMENTATION BlurdyBlurp POP3 server
438 (the following lines are broken for editorial clarity only)
439 C: BAYFQU5EUkVXLkNNVS5FRFUAOCCXeMiVyFe9K6Nwne7+sPLgIoF9YQ5ePfxUsMlJAf
440 C7aoNySU8nrqS9m8JAddsUeuyc5HFXXovaKLrZNo2bTLH0Lyolwy0W9ryJDojbKmHy
443 C: vSAF7ha6qotK2UHUgKlsEA==
444 S: +OK Maildrop locked and ready
445 ... at this point a security layer has been established and additional
446 commands and responses proceed within it ...
451 The following syntax specification uses the Augmented Backus-Naur
452 Form notation as specified in [ABNF].
454 Except as noted otherwise, all alphabetic characters are case-
455 insensitive. The use of upper or lower case characters to define
456 token strings is for editorial clarity only. Implementations MUST
457 accept these strings in a case-insensitive fashion.
460 UPALPHA = %x41-5A ;; Uppercase: A-Z
462 LOALPHA = %x61-7A ;; Lowercase: a-z
464 ALPHA = UPALPHA / LOALPHA ;; case insensitive
466 DIGIT = %x30-39 ;; Digits 0-9
468 AUTH_CHAR = ALPHA / DIGIT / "-" / "_"
470 auth_type = 1*20AUTH_CHAR
472 auth_command = "AUTH" SPACE auth_type [SPACE (base64 / "=")]
473 *(CRLF [base64]) CRLF
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487 base64 = base64_terminal /
488 ( 1*(4base64_CHAR) [base64_terminal] )
490 base64_char = UPALPHA / LOALPHA / DIGIT / "+" / "/"
493 base64_terminal = (2base64_char "==") / (3base64_char "=")
495 continue_req = "+" SPACE [base64] CRLF
497 CR = %x0C ;; ASCII CR, carriage return
501 LF = %x0A ;; ASCII LF, line feed
503 SPACE = %x20 ;; ASCII SP, space
506 6. Security Considerations
508 Security issues are discussed throughout this memo.
510 Before the [SASL] negotiation has begun, any protocol interactions
511 are performed in the clear and may be modified by an active
512 attacker. For this reason, clients and servers MUST discard any
513 knowledge obtained prior to the start of the SASL negotiation upon
514 the establishment of a security layer.
516 7. IANA Considerations
518 This document requests that the IANA update the entry for the "pop"
519 SASL protocol name to point at this document.
521 This document requests that the IANA update the entry for the SASL
522 POP3 capability to be as defined in Section 3 of this document.
524 8. Intellectual Property Rights
526 The IETF takes no position regarding the validity or scope of any
527 intellectual property or other rights that might be claimed to
528 pertain to the implementation or use of the technology described in
529 this document or the extent to which any license under such rights
530 might or might not be available; neither does it represent that it
531 has made any effort to identify any such rights. Information on the
532 IETF's procedures with respect to rights in standards-track and
533 standards-related documentation can be found in BCP-11. Copies of
534 claims of rights made available for publication and any assurances
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544 POP3 SASL Authentication Mechanism October, 2003
547 of licenses to be made available, or the result of an attempt made
548 to obtain a general license or permission for the use of such
549 proprietary rights by implementors or users of this specification
550 can be obtained from the IETF Secretariat.
552 The IETF invites any interested party to bring to its attention any
553 copyrights, patents or patent applications, or other proprietary
554 rights which may cover technology that may be required to practice
555 this standard. Please address the information to the IETF Executive
560 Copyright (C) The Internet Society (2003). All Rights Reserved.
562 This document and translations of it may be copied and furnished to
563 others, and derivative works that comment on or otherwise explain it
564 or assist in its implementation may be prepared, copied, published
565 and distributed, in whole or in part, without restriction of any
566 kind, provided that the above copyright notice and this paragraph
567 are included on all such copies and derivative works. However, this
568 document itself may not be modified in any way, such as by removing
569 the copyright notice or references to the Internet Society or other
570 Internet organizations, except as needed for the purpose of
571 developing Internet standards in which case the procedures for
572 copyrights defined in the Internet Standards process must be
573 followed, or as required to translate it into languages other than
576 This document and the information contained herein is provided on an
577 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
578 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
579 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
580 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
581 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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604 POP3 SASL Authentication Mechanism October, 2003
609 The following documents contain normative definitions or
610 specifications that are necessary for correct understanding of this
613 [BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
614 Encodings", RFC 3548, July 2003.
616 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
617 Requirement Levels", BCP 14, RFC 2119, March 1997
619 [POP3] Myers, J. & Rose, M., "Post Office Protocol - Version 3",
620 STD 53, RFC 1939, May 1996.
622 [POP3-EXT] Gellens, R., Newman, C. & Lundblade, L., "POP3 Extension
623 Mechanism", RFC 2449, November 1998.
625 [POP3-TLS] Newman, C. "Using TLS with IMAP, POP3, and ACAP", RFC 2595,
628 [SASL] Myers, J., "Simple Authentication and Security Layer
629 (SASL)", RFC 2222, October 1997.
631 The following references are for informational purposes only:
633 [POP3-AUTH] Myers, J., "POP3 AUTHentication Command", RFC 1734, January
636 [TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
639 11. Changes From RFC 1734 and RFC 2449
641 1. The SASL-based semantics defined in RFC 2449 are now
642 normative for the AUTH extension.
644 2. Clarifications and examples of the proper behavior of
645 initial client response handling.
647 3. Minimum requirement of support for TLS+PLAIN.
649 4. Clarify ordering of TLS and SASL security layers.
651 5. Update references to newer versions of various
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664 POP3 SASL Authentication Mechanism October, 2003
667 6. Clarify that the mechanism list can change.
669 7. General other editorial clarifications.
671 12. Author's Address:
674 Carnegie Mellon, Andrew Systems Group
683 The author would like to acknowledge the contributions of John Myers
684 and other contributors to RFC 1734, on which this document draws
687 The author would also like to thank Ken Murchison and Mark Crispin
688 for the time they devoted to reviewing early drafts of this
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