INTERNET-DRAFT Editor: Kurt D. Zeilenga Intended Category: Standards Track OpenLDAP Foundation Expires in six months 4 November 2002 Obsoletes: RFC 2245 Anonymous SASL Mechanism Status of Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 . 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 . The list of Internet-Draft Shadow Directories can be accessed at . Copyright 2002, The Internet Society. All Rights Reserved. Please see the Copyright section near the end of this document for more information. 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 Zeilenga Anonymous SASL Mechanism [Page 1] INTERNET-DRAFT draft-zeilenga-sasl-anon-01.txt 4 November 2002 plaintext 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". 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 [UTF8] encoded printable characters from the Universal Character Set ([UCS]). The trace information 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 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. This mechanism does not provide a security layer. The formal grammar for the client message using Augmented BNF [ABNF] follows. message = [ email / token ] UTF1 = %x00-3F / %x41-7F ;; less '@' (U+0040) UTF2 = %xC0-DF 1(UTF0) UTF3 = %xE0-EF 2(UTF0) UTF4 = %xF0-F7 3(UTF0) UTF5 = %xF8-FB 4(UTF0) UTF6 = %xFC-FD 5(UTF0) UTF0 = %x80-BF TCHAR = UTF1 / UTF2 / UTF3 / UTF3 / UTF4 / UTF5 / UTF6 ;; any UTF-8 encoded UCS printable character ;; except '@' (U+0040) email = addr-spec Zeilenga Anonymous SASL Mechanism [Page 2] INTERNET-DRAFT draft-zeilenga-sasl-anon-01.txt 4 November 2002 ;; 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 The following characters are considered non-printable: - control characters: U+0000..U+001F, U+007F..U+009F; - replacement character: U+FFFD; and - special characters and non-character: U+FEFF, U+FFFE, U+FFFF. 2. 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=DIGEST-MD5 AUTH=ANONYMOUS S: A001 OK done C: A002 AUTHENTICATE ANONYMOUS S: + C: c2lyaGM= S: A003 OK Welcome, trace information has been logged. 3. 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 Zeilenga Anonymous SASL Mechanism [Page 3] INTERNET-DRAFT draft-zeilenga-sasl-anon-01.txt 4 November 2002 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 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 Zeilenga Anonymous SASL Mechanism [Page 4] INTERNET-DRAFT draft-zeilenga-sasl-anon-01.txt 4 November 2002 an explicit login step. General SASL security considerations apply to this mechanism. 4. 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: iana@iana.org 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 Neuman Intended usage: COMMON Author/Change controller: IESG Note: Updates existing entry for ANONYMOUS 5. Acknowledgement This document is a revision of RFC 2245 by Chris Newman. 6. Normative 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. [SASL] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222bis (a work in progress). [UCS] Universal Multiple-Octet Coded Character Set (UCS) - Architecture and Basic Multilingual Plane, ISO/IEC 10646-1 : 1993. [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. Zeilenga Anonymous SASL Mechanism [Page 5] INTERNET-DRAFT draft-zeilenga-sasl-anon-01.txt 4 November 2002 7. Informative References [IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 2060, December 1996. [IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL) MECHANISMS", http://www.iana.org/assignments/sasl- mechanisms. 8. Editor's Address Kurt Zeilenga OpenLDAP Foundation Email: kurt@OpenLDAP.org Appendix A. Changes since RFC 2245 This appendix is non-normative. RFC 2245 allows the client to send optional trace information in the form of a human readable string, but did not define the character encoding nor character set to be used. This document mandates use of UTF-8 encoded UCS printable characters. The grammar was updated to reflect this. Additionally, a number of editorial changes were made. Full Copyright Statement Copyright 2002, The Internet Society. 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. Zeilenga Anonymous SASL Mechanism [Page 6] INTERNET-DRAFT draft-zeilenga-sasl-anon-01.txt 4 November 2002 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 AUTHORS, 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. Zeilenga Anonymous SASL Mechanism [Page 7]