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Network Working Group                                   K. Zeilenga, Ed.Request for Comments: 4505                           OpenLDAP FoundationObsoletes:2245                                                June 2006Category: Standards TrackAnonymous Simple Authentication and Security Layer (SASL) MechanismStatus 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).Abstract   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)   framework.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   method.   This mechanism does not provide a security layer.   This document replacesRFC 2245.  Changes sinceRFC 2245 are detailed   inAppendix A.Zeilenga                    Standards Track                     [Page 1]

RFC 4505                Anonymous SASL Mechanism               June 20062.  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 inSection 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   specification.      message     = [ email / token ]                    ;; to be prepared in accordance withSection 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.Zeilenga                    Standards Track                     [Page 2]

RFC 4505                Anonymous SASL Mechanism               June 20063.  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   inAppendix 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].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 "+ ").Zeilenga                    Standards Track                     [Page 3]

RFC 4505                Anonymous SASL Mechanism               June 2006   In this example, the trace information 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.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.Zeilenga                    Standards Track                     [Page 4]

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: SeeRFC 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 ANONYMOUSZeilenga                    Standards Track                     [Page 5]

RFC 4505                Anonymous SASL Mechanism               June 2006   The [StringPrep] profile "trace", first defined in this RFC, has been   registered:      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 ofRFC 2245 by Chris Newman.  Portions of   the grammar defined inSection 1 were borrowed fromRFC 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                2001.   [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"                (http://www.unicode.org/reports/tr28/).   [UTF-8]      Yergeau, F., "UTF-8, a transformation format of ISO                10646",RFC 3629 (also STD 63), November 2003.Zeilenga                    Standards Track                     [Page 6]

RFC 4505                Anonymous SASL Mechanism               June 20069.  Informative References   [IMAP4]      Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION                4rev1",RFC 3501, March 2003.   [IANA-SASL]  IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)                MECHANISMS", <http://www.iana.org/assignments/sasl-mechanisms>.Zeilenga                    Standards Track                     [Page 7]

RFC 4505                Anonymous SASL Mechanism               June 2006Appendix A.  Changes sinceRFC 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.orgZeilenga                    Standards Track                     [Page 8]

RFC 4505                Anonymous SASL Mechanism               June 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Zeilenga                    Standards Track                     [Page 9]