INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standards Track            OpenLDAP Foundation
Expires in six months                              8 January 2003
Obsoletes: RFC 2245

                       The Anonymous SASL Mechanism

Status of Memo

  This document is an Internet-Draft and is in full conformance with all
  provisions of Section 10 of RFC2026.

  This document is intended to be, after appropriate review and
  revision, submitted to the RFC Editor as a Standards Track document.
  Distribution of this memo is unlimited.  Technical discussion of this
  document will take place on the IETF SASL mailing list
  <>.  Please send editorial comments directly to the
  document editor <>.

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  Copyright 2002, The Internet Society.  All Rights Reserved.

  Please see the Copyright section near the end of this document for
  more information.


  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

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  plaintext login commands are not permitted in new IETF protocols, a
  new way to provide anonymous login is needed within the context of the
  Simple Authentication and Security Layer (SASL) framework.


  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  document are to be interpreted as described in [KEYWORDS].

1. Anonymous SASL mechanism

  This document defines an anonymous mechanism for the Simple
  Authentication and Security Layer ([SASL]) framework.  The name
  associated with this mechanism is "ANONYMOUS".

  This document replaces RFC 2245.  Changes since RFC 2245 are detailed
  in Appendix A.

  The mechanism consists of a single message from the client to the
  server.  The client sends optional trace information in the form of a
  string of [UTF-8] encoded [Unicode] characters prepared in accordance
  with [Stringprep] and the "plain" stringprep profile defined in
  Section 2 of this document.  The trace information should take one of
  three forms: an Internet email address, an opaque string which does
  not contain the '@' (U+0040) character and can be interpreted by the
  system administrator of the client's domain, or nothing.  For privacy
  reasons, an Internet email address or other information identifying
  the user should only be used with permission from the user.

  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.

  This mechanism does not provide a security layer.

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

      message     = [ email / token ]
                       ;; MUST be prepared in accordance with Section 2

      UTF1        = %x00-3F / %x41-7F ;; less '@' (U+0040)
      UTF2        = %xC0-DF 1(UTF0)
      UTF3        = %xE0-EF 2(UTF0)
      UTF4        = %xF0-F7 3(UTF0)

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      UTF5        = %xF8-FB 4(UTF0)
      UTF6        = %xFC-FD 5(UTF0)
      UTF0        = %x80-BF

      TCHAR       = UTF1 / UTF2 / UTF3 / UTF3 / UTF4 / UTF5 / UTF6
                    ;; any UTF-8 encoded Unicode character
                    ;; except '@' (U+0040)

      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

      Note to implementors:
      The <token> production is restricted to 255 UTF-8 encoded
      characters.  As the encoding of a characters use a sequence of 1
      to 6 octets, a token may be long as 1530 octets.

2. The plain "stringprep" profile

  This section defines the "plain" profile of [Stringprep].  This
  profile is designed for use with the SASL ANONYMOUS Mechanism.
  Specifically, the client MUST prepare the <message> production in
  accordance with this profile.

  The character repertiore of this profile is Unicode 3.2 [Unicode].

  No mapping is required by this profile.

  No Unicode normalization is required by this profile.

  Characters from the following tables of [Stringprep] are prohibited:
      - C.2.1 (ASCII control characters)
      - C.2.2 (Non-ASCII control characters)
      - C.3 (Private use characters)
      - C.4 (Non-character code points)
      - C.5 (Surrogate codes)
      - C.6 (Inappropriate for plain text)
      - C.8 (Change display properties or are deprecated)
      - C.9 (Tagging characters)

  No additional characters are prohibited.

  This profile requires bidirectional character checking per Section 6
  of [Stringprep].

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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: A001 OK done
      S: +
      C: c2lyaGM=
      S: A003 OK Welcome, trace information has been logged.

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.

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  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

  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
  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(2) 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.

  General [SASL] security considerations apply to this mechanism.

  [Stringprep] security considerations as well as [Unicode], security
  considerations discussed in [Stringprep], apply to this mechanism.

5. IANA Considerations

  It is requested that the SASL Mechanism registry [IANA-SASL] entry for
  the ANONYMOUS mechanism be updated to reflect that this document now
  provides its technical specification.

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      Subject: Updated Registration of SASL mechanism ANONYMOUS

      SASL mechanism name: ANONYMOUS
      Security considerations: See RFC XXXX.
      Published specification (optional, recommended): RFC XXXX
      Person & email address to contact for further information:
           Kurt Zeilenga <>
           Chris Neuman <>
      Intended usage: COMMON
      Author/Change controller: IESG <>
      Note: Updates existing entry for ANONYMOUS

  It is requested that the "stringprep" [RFC3454] profile "plain", first
  defined in this RFC, be registered:

      Subject: Initial Registration of Stringprep "plain" profile

      Stringprep profile: plain
      Published specification: RFC XXXX
      Person & email address to contact for further information:
          Kurt Zeilenga <>

6. Acknowledgement

  This document is a revision of RFC 2245 by Chris Newman.

7. Normative 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.

  [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997

  [SASL]       Myers, J., "Simple Authentication and Security Layer
               (SASL)", RFC 2222bis (a work in progress).

  [Stringprep] P. Hoffman, M. Blanchet, "Preparation of
               Internationalized Strings ('stringprep')", RFC 3454,
               December 2002.

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  [Unicode]    The Unicode Consortium. The Unicode Standard, Version
               3.2.0 is defined by The Unicode Standard, Version 3.0
               (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
               as amended by the Unicode Standard Annex #27: Unicode 3.1
               ( and by the Unicode
               Standard Annex #28: Unicode 3.2

  [UTF-8]      Yergeau, F., "UTF-8, a transformation format of ISO
               10646", RFC 2279, January 1998.

8. Informative References

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


9. Editor's Address

  Kurt Zeilenga
  OpenLDAP Foundation


Appendix A.  Changes since RFC 2245

  This appendix is non-normative.

  RFC 2245 allows the client to send optional trace information in the
  form of a human readable string.  RFC 2245 restricted this string to
  US-ASCII.  As the Internet is international, this document uses a
  string restricted to UTF-8 encoded Unicode "plain" characters.  A
  "stringprep" profile is defined to precisely define which Unicode
  characters are allowed in this string.  While the string remains
  restricted to 255 characters, the encoded length of each character may
  now range from 1 to 6 octets.

  Additionally, a number of editorial changes were made.

Full Copyright Statement

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  Copyright 2002, The Internet Society.  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
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  developing Internet standards in which case the procedures for
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  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

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