Internet DRAFT - draft-ietf-sasl-anon


INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standards Track            OpenLDAP Foundation
Expires: August 2005                             21 February 2005
Obsoletes: RFC 2245

                       The Anonymous SASL Mechanism

Status of Memo

  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 <>.

  By submitting this Internet-Draft, I accept the provisions of Section
  4 of RFC 3667.  By submitting this Internet-Draft, I certify that any
  applicable patent or other IPR claims of which I am aware have been
  disclosed, or will be disclosed, and any of which I become aware will
  be disclosed, in accordance with RFC 3668.

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  Copyright (C) The Internet Society (2005).  All Rights Reserved.

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

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  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
  plain text 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.

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".

  Unlike many other SASL mechanisms whose purpose is to authenticate and
  identify the user to a server, the purpose of this SASL mechanism is
  to allow the user to gain access to services or resources without
  requiring the user to establish or otherwise disclose their identity
  to the server.  That is, this mechanism provides an anonymous login

  This mechanism does not provide a security layer.

  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 may include in this message trace information in
  the form of a string of [UTF-8] encoded [Unicode] characters prepared
  in accordance with [StringPrep] and the "trace" stringprep profile
  defined in Section 2 of this document.  The trace information, which
  has no semantical value, should take one of two 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.  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.

  A formal grammar for the client message using Augmented BNF [ABNF] is
  provide below as a tool for understanding this technical

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

      UTF1        = %x00-3F / %x41-7F ;; less '@' (U+0040)
      UTF2        = %xC2-DF UTF0
      UTF3        = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
                    %xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
      UTF4        = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
                    %xF4 %x80-8F 2(UTF0)
      UTF0        = %x80-BF

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

      email       = addr-spec
                    ;; as defined in [IMAIL]

      token       = 1*255TCHAR

  Note to implementors:
      The <token> production is restricted to 255 UTF-8 encoded Unicode
      characters.   As the encoding of a characters uses a sequence of 1
      to 4 octets, a token may be long as 1020 octets.

2. The "trace" profile of "Stringprep"

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

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

  No mapping is required by this profile.

  No Unicode normalization is required by this profile.

  The list of unassigned code points for this profile is that provided
  in appendix A of [StringPrep].  Unassigned code points are not

  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)

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      - C.5 (Surrogate codes)
      - C.6 (Inappropriate for plain text)
      - C.8 (Change display properties are deprecated)
      - C.9 (Tagging characters)

  No additional characters are prohibited.

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

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.  Additionally, protocols with SASL profiles permitting an
  initial client response will be able to avoid the extra round trip
  below (the server response with an empty "+ ").

  In this example, the trace information 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 services and/or resources 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.

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  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 reduce the priority of anonymous users or
  limit their resource usage.

  While servers may impose a limit on the number of anonymous users, it
  is noted that such limits enable denial of service attacks and should
  be used with caution.

  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 investigating
  abuse need to realize this trace 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.  Anyone who accesses an anonymous archive on a sensitive
  subject (e.g. sexual abuse) likely 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 information --
  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 data security services.

  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.

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  [StringPrep] security considerations as well as [Unicode] security
  considerations discussed in [StringPrep] apply to this mechanism.
  UTF-8 [RFC3629] security considerations also apply.

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.

      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 Newman <>
      Intended usage: COMMON
      Author/Change controller: IESG <>
      Note: Updates existing entry for ANONYMOUS

  It is requested that the [Stringprep] profile "trace", first defined
  in this RFC, be registered:

      Subject: Initial Registration of Stringprep "trace" profile

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

6. Acknowledgment

  This document is a revision of RFC 2245 by Chris Newman.  Portions of
  the grammar defined in Section 1 were borrowed from RFC 3629 by
  Francois Yergeau.

  This document is a product of the IETF SASL WG.

7. Normative References

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

  [IMAIL]       Resnick, P., Ed., "Internet Message Format", RFC 2822,
                April 2001.

  [SASL]        Melnikov, A. (Editor), "Simple Authentication and
                Security Layer (SASL)",
                draft-ietf-sasl-rfc2222bis-xx.txt, a work in progress.

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

  [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 3629 (also STD 63), November 2003.

8. Informative References

  [IMAP4]       Crispin, M., "Internet Message Access Protocol - Version
                4rev1", RFC 3501, March 2003.


9. Editor's Address

  Kurt D. Zeilenga
  OpenLDAP Foundation


Appendix A.  Changes since RFC 2245

  This appendix is non-normative.

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  RFC 2245 allows the client to include 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 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 4 octets.

  Additionally, a number of editorial changes were made.

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Full Copyright

  Copyright (C) The Internet Society (2005).  This document is subject
  to the rights, licenses and restrictions contained in BCP 78, and
  except as set forth therein, the authors retain all their rights.

  This document and the information contained herein are provided on an

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