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INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standards Track            OpenLDAP Foundation
Expires: April 2005                               25 October 2004
Obsoletes: RFC 2245




                       The Anonymous SASL Mechanism
                      <draft-ietf-sasl-anon-04.txt>




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
  <address@hidden>.  Please send editorial comments directly to the
  document editor <address@hidden>.


  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.


  Internet-Drafts are working documents of the Internet Engineering Task
  Force (IETF), its areas, and its working groups. Note that other
  groups may also distribute working documents as Internet-Drafts.


  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time. It is inappropriate to use Internet-Drafts as reference material
  or to cite them other than as "work in progress."


  The list of current Internet-Drafts can be accessed at
  <http://www.ietf.org/ietf/1id-abstracts.txt>.  The list of
  Internet-Draft Shadow Directories can be accessed at
  <http://www.ietf.org/shadow.html>.


  Copyright (C) The Internet Society (2004).  All Rights Reserved.


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







Zeilenga                Anonymous SASL Mechanism                [Page 1]
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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
  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.



Conventions


  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  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".


  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
  method.


  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 sends optional 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 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.





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  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
  specification.


      message     = [ email / token ]
                       ;; MUST 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], 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 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 MUST 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.





Zeilenga                Anonymous SASL Mechanism                [Page 3]
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  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
  prohibited.


  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].



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 user's opaque identification token is "sirhc".


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





Zeilenga                Anonymous SASL Mechanism                [Page 4]
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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.


  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 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 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




Zeilenga                Anonymous SASL Mechanism                [Page 5]
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  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.
  UTF-8 [RFC3679] 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.


      To: address@hidden
      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 <address@hidden>
           Chris Neuman <address@hidden>
      Intended usage: COMMON
      Author/Change controller: IESG <address@hidden>
      Note: Updates existing entry for ANONYMOUS



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


      To: address@hidden
      Subject: Initial Registration of Stringprep "trace" profile


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






Zeilenga                Anonymous SASL Mechanism                [Page 6]
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6. Acknowledgment


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