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Network Working Group                                   K. Zeilenga, Ed.
Request for Comments: 4505                           OpenLDAP Foundation
Obsoletes: 2245                                                June 2006
Category: Standards Track

  Anonymous Simple Authentication and Security Layer (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 (2006).


   On the Internet, it is common practice 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 using
   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)

1.  Introduction

   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.

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RFC 4505                Anonymous SASL Mechanism               June 2006

2.  The Anonymous Mechanism

   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 3 of this document.  The trace information, which
   has no semantical value, should take one of two forms: an Internet
   email address, or an opaque string that does not contain the '@'
   (U+0040) character and that 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 that 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
   provided below as a tool for understanding this technical

      message     = [ email / token ]
                    ;; to be prepared in accordance with Section 3

      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 as long as 1020 octets.

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RFC 4505                Anonymous SASL Mechanism               June 2006

3.  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)
      - 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].

4.  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 "+ ").

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RFC 4505                Anonymous SASL Mechanism               June 2006

   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.

5.  Security Considerations

   The ANONYMOUS mechanism grants access to services and/or resources by
   anyone.  For this reason, it should be disabled by default so that
   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 exchange
   information anonymously.  Servers that 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,
   note 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 that this trace information may be falsified.

   A client that 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 the explicit
   permission of the user and should offer the option of supplying no
   trace information, thus only exposing the source IP address and time.

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RFC 4505                Anonymous SASL Mechanism               June 2006

   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
   that view or alter the data transferred.  Clients and servers are
   encouraged to support external data security services.

   Protocols that fail to require an explicit anonymous login are more
   susceptible to break-ins given certain common implementation
   techniques.  Specifically, Unix servers that 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 that 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 and [Unicode] security
   considerations discussed in [StringPrep] apply to this mechanism.
   [UTF-8] security considerations also apply.

6.  IANA Considerations

   The SASL Mechanism registry [IANA-SASL] entry for the ANONYMOUS
   mechanism has been updated by the IANA to reflect that this document
   now provides its technical specification.

      To: iana@iana.org
      Subject: Updated Registration of SASL mechanism ANONYMOUS

      SASL mechanism name: ANONYMOUS
      Security considerations: See RFC 4505.
      Published specification (optional, recommended): RFC 4505
      Person & email address to contact for further information:
           Kurt Zeilenga <Kurt@OpenLDAP.org>
           Chris Newman <Chris.Newman@sun.com>
      Intended usage: COMMON
      Author/Change controller: IESG <iesg@ietf.org>
      Note: Updates existing entry for ANONYMOUS

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RFC 4505                Anonymous SASL Mechanism               June 2006

   The [StringPrep] profile "trace", first defined in this RFC, has been

      To: iana@iana.org
      Subject: Initial Registration of Stringprep "trace" profile

      Stringprep profile: trace
      Published specification: RFC 4505
      Person & email address to contact for further information:
          Kurt Zeilenga <kurt@openldap.org>

7.  Acknowledgement

   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.

8.  Normative References

   [ABNF]       Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 4234, October 2005.

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

   [SASL]       Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
                Authentication and Security Layer (SASL)", RFC 4422,
                June 2006.

   [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" (http://www.unicode.org/reports/tr27/) 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.

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RFC 4505                Anonymous SASL Mechanism               June 2006

9.  Informative References

                4rev1", RFC 3501, March 2003.

                MECHANISMS", <http://www.iana.org/assignments/sasl-

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RFC 4505                Anonymous SASL Mechanism               June 2006

Appendix A.  Changes since RFC 2245

   This appendix is non-normative.

   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.

Editor's Address

   Kurt D. Zeilenga
   OpenLDAP Foundation

   EMail: Kurt@OpenLDAP.org

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RFC 4505                Anonymous SASL Mechanism               June 2006

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