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Network Working Group C. Newman Request for Comments: 2245 Innosoft Category: Standards Track November 1997

                      Anonymous SASL 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 Internet Society (1997).  All Rights Reserved.

Abstract

 It is common practice on the Internet to permit anonymous access to
 various services.  Traditionally, this has been done with a plain
 text password mechanism using "anonymous" as the user name and
 optional trace information, such as an email address, as the
 password.  As plaintext login commands are not permitted in new IETF
 protocols, a new way to provide anonymous login is needed within the
 context of the SASL [SASL] framework.

1. Conventions Used in this Document

 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
 in this document are to be interpreted as defined in "Key words for
 use in RFCs to Indicate Requirement Levels" [KEYWORDS].

2. Anonymous SASL mechanism

 The mechanism name associated with anonymous access is "ANONYMOUS".
 The mechanism consists of a single message from the client to the
 server.  The client sends optional trace information in the form of a
 human readable string.  The trace information should take one of
 three forms: an Internet email address, an opaque string which does
 not contain the '@' character and can be interpreted by the system
 administrator of the client's domain, or nothing.  For privacy
 reasons, an Internet email address should only be used with
 permission from the user.

Newman Standards Track [Page 1]  RFC 2245 Anonymous SASL Mechanism November 1997

 A server which permits anonymous access will announce support for the
 ANONYMOUS mechanism, and allow anyone to log in using that mechanism,
 usually with restricted access.

 The formal grammar for the client message using Augmented BNF [ABNF]
 follows.

 message         = [email / token]

 TCHAR           = %x20-3F / %x41-7E
                   ;; any printable US-ASCII character except '@'

 email           = addr-spec
                   ;; as defined in [IMAIL], except with no free
                   ;; insertion of linear-white-space, and the
                   ;; local-part MUST either be entirely enclosed in
                   ;; quotes or entirely unquoted

 token           = 1*255TCHAR

3. Example

 Here is a sample anonymous login between an IMAP client and server.
 In this example, "C:" and "S:" indicate lines sent by the client and
 server respectively.  If such lines are wrapped without a new "C:" or
 "S:" label, then the wrapping is for editorial clarity and is not
 part of the command.

 Note that this example uses the IMAP profile [IMAP4] of SASL.  The
 base64 encoding of challenges and responses, as well as the "+ "
 preceding the responses are part of the IMAP4 profile, not part of
 SASL itself.  Newer profiles of SASL will include the client message
 with the AUTHENTICATE command itself so the extra round trip below
 (the server response with an empty "+ ") can be eliminated.

 In this example, the user's opaque identification token is "sirhc".

      S: * OK IMAP4 server ready
      C: A001 CAPABILITY
      S: * CAPABILITY IMAP4 IMAP4rev1 AUTH=CRAM-MD5 AUTH=ANONYMOUS
      S: A001 OK done
      C: A002 AUTHENTICATE ANONYMOUS
      S: +
      C: c2lyaGM=
      S: A003 OK Welcome, trace information has been logged.

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4. Security Considerations

 The anonymous mechanism grants access to information by anyone.  For
 this reason it should be disabled by default so the administrator can
 make an explicit decision to enable it.

 If the anonymous user has any write privileges, a denial of service
 attack is possible by filling up all available space.  This can be
 prevented by disabling all write access by anonymous users.

 If anonymous users have read and write access to the same area, the
 server can be used as a communication mechanism to anonymously
 exchange information.  Servers which accept anonymous submissions
 should implement the common "drop box" model which forbids anonymous
 read access to the area where anonymous submissions are accepted.

 If the anonymous user can run many expensive operations (e.g., an
 IMAP SEARCH BODY command), this could enable a denial of service
 attack.  Servers are encouraged to limit the number of anonymous
 users and reduce their priority or limit their resource usage.

 If there is no idle timeout for the anonymous user and there is a
 limit on the number of anonymous users, a denial of service attack is
 enabled.  Servers should implement an idle timeout for anonymous
 users.

 The trace information is not authenticated so it can be falsified.
 This can be used as an attempt to get someone else in trouble for
 access to questionable information.  Administrators trying to trace
 abuse need to realize this information may be falsified.

 A client which uses the user's correct email address as trace
 information without explicit permission may violate that user's
 privacy.  Information about who accesses an anonymous archive on a
 sensitive subject (e.g., sexual abuse) has strong privacy needs.
 Clients should not send the email address without explicit permission
 of the user and should offer the option of supplying no trace token
 -- thus only exposing the source IP address and time.  Anonymous
 proxy servers could enhance this privacy, but would have to consider
 the resulting potential denial of service attacks.

 Anonymous connections are susceptible to man in the middle attacks
 which view or alter the data transferred.  Clients and servers are
 encouraged to support external integrity and encryption mechanisms.

 Protocols which fail to require an explicit anonymous login are more
 susceptible to break-ins given certain common implementation
 techniques.  Specifically, Unix servers which offer user login may

Newman Standards Track [Page 3]  RFC 2245 Anonymous SASL Mechanism November 1997

 initially start up as root and switch to the appropriate user id
 after an explicit login command.  Normally such servers refuse all
 data access commands prior to explicit login and may enter a
 restricted security environment (e.g., the Unix chroot function) for
 anonymous users.  If anonymous access is not explicitly requested,
 the entire data access machinery is exposed to external security
 attacks without the chance for explicit protective measures.
 Protocols which offer restricted data access should not allow
 anonymous data access without an explicit login step.

5. References

 [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
 Specifications: ABNF", RFC 2234, November 1997.

 [IMAIL] Crocker, D., "Standard for the Format of Arpa Internet Text
 Messages", STD 11, RFC 822, August 1982.

 [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
 4rev1", RFC 2060, December 1996.

 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", RFC 2119, March 1997.

 [SASL] Myers, J., "Simple Authentication and Security Layer (SASL)",
 RFC 2222, October 1997.

6. Author's Address

 Chris Newman
 Innosoft International, Inc.
 1050 Lakes Drive
 West Covina, CA 91790 USA

 Email: chris.newman@innosoft.com

Newman Standards Track [Page 4]  RFC 2245 Anonymous SASL Mechanism November 1997

7. Full Copyright Statement

 Copyright (C) The Internet Society (1997).  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.

 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.

 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.

Newman Standards Track [Page 5]