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Network Working Group                                          C. NewmanRequest for Comments: 2245                                      InnosoftCategory: Standards Track                                  November 1997Anonymous 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 (1997).  All Rights Reserved.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 plaintext login commands are not permitted in new IETF   protocols, a new way to provide anonymous login is needed within the   context of the SASL [SASL] framework.1. Conventions Used in this Document   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"   in this document are to be interpreted as defined in "Key words for   use in RFCs to Indicate Requirement Levels" [KEYWORDS].2. Anonymous SASL mechanism   The mechanism name associated with anonymous access is "ANONYMOUS".   The mechanism consists of a single message from the client to the   server.  The client sends optional trace information in the form of a   human readable string.  The trace information should take one of   three forms: an Internet email address, an opaque string which does   not contain the '@' character and can be interpreted by the system   administrator of the client's domain, or nothing.  For privacy   reasons, an Internet email address should only be used with   permission from the user.Newman                      Standards Track                     [Page 1]

RFC 2245                Anonymous SASL Mechanism           November 1997   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.   The formal grammar for the client message using Augmented BNF [ABNF]   follows.   message         = [email / token]   TCHAR           = %x20-3F / %x41-7E                     ;; any printable US-ASCII character except '@'   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*255TCHAR3. 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: * CAPABILITY IMAP4 IMAP4rev1 AUTH=CRAM-MD5 AUTH=ANONYMOUS        S: A001 OK done        C: A002 AUTHENTICATE ANONYMOUS        S: +        C: c2lyaGM=        S: A003 OK Welcome, trace information has been logged.Newman                      Standards Track                     [Page 2]

RFC 2245                Anonymous SASL Mechanism           November 19974. 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.   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   users.   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 mayNewman                      Standards Track                     [Page 3]

RFC 2245                Anonymous SASL Mechanism           November 1997   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 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.5. 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.   [IMAP4] Crispin, M., "Internet Message Access Protocol - Version   4rev1",RFC 2060, December 1996.   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate   Requirement Levels",RFC 2119, March 1997.   [SASL] Myers, J., "Simple Authentication and Security Layer (SASL)",RFC 2222, October 1997.6. Author's Address   Chris Newman   Innosoft International, Inc.   1050 Lakes Drive   West Covina, CA 91790 USA   Email: chris.newman@innosoft.comNewman                      Standards Track                     [Page 4]

RFC 2245                Anonymous SASL Mechanism           November 19977.  Full Copyright Statement   Copyright (C) The Internet Society (1997).  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   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, 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   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS 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.Newman                      Standards Track                     [Page 5]