Network Working Group C. Daboo Internet Draft: SIEVE spamtest and virustest Extensions Document: draft-daboo-sieve-spamtest-02.txt March 2003 SIEVE spamtest and virustest Extensions Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 Notice Copyright (C) The Internet Society 2003. All Rights Reserved. Daboo Expires September 2003 [Page 1] Internet Draft SIEVE spamtest and virustest Extensions March 2003 Table of Contents 1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 Conventions Used in This Document . . . . . . . . . . . . . . 2 3 Change History . . . . . . . . . . . . . . . . . . . . . . . 2 4 Introduction and Overview . . . . . . . . . . . . . . . . . . 2 5 SIEVE Extensions . . . . . . . . . . . . . . . . . . . . . . 3 5.1 spamtest . . . . . . . . . . . . . . . . . . . . . . . . . 3 5.2 virustest . . . . . . . . . . . . . . . . . . . . . . . . 4 6 Security Considerations . . . . . . . . . . . . . . . . . . . 5 7 Normative References . . . . . . . . . . . . . . . . . . . . 5 8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . 6 10 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 6 1 Abstract The SIEVE [SIEVE] "spamtest" and "virustest" extensions permit users to use simple, portable commands for spam and virus tests on email messages. Each extension provides a new test using matches against numeric 'scores'. It is the responsibility of the underlying SIEVE implementation to do the actual checks that result in values returned by the tests. 2 Conventions Used in This Document Conventions for notations are as in [SIEVE] section 1.1, including use of [KEYWORDS]. The term 'spam' is used in this document to refer to unsolicited or unwanted email messages. This document does not attempt to define what exactly constitutes spam, or how it should be identified, or what actions should be taken when detected. The term 'virus' is used in this document to refer to any type of message whose content can cause malicious damage. This document does not attempt to define what exactly constitutes a virus, or how it should be identified, or what actions should be taken when detected. 3 Change History Changes from -01 to -02: 1 Fixed syntax in examples. 2 Updated references section to normative/informative. Changes from -00 to -01: 3 Changed so that tests use standard SIEVE syntax. 4 Added requirement for relation extension for numeric comparisons. 5 Changed spamtest numeric range to 0->10. Daboo Expires September 2003 [Page 2] Internet Draft SIEVE spamtest and virustest Extensions March 2003 4 Introduction and Overview SIEVE scripts are frequently being used to do spam and virus filtering based on either implicit script tests (e.g. tests for 'black-listed' senders directly encoded in the SIEVE script), or via testing messages modified by some external spam or virus checker that handled the message prior to SIEVE. The use of third-party spam and virus checker tools poses a problem since each tool has its own way of indicating the result of its checks. These usually take the form of a header added to the message, the content of which indicates the status using some syntax defined by the particular tool. Each user has to then create their own SIEVE scripts to match the contents of these headers to do filtering. This requires the script to stay in synchronisation with the third party tool as it gets updated or perhaps replaced with another. Thus scripts become tied to specific environments, and lose portability. The purpose of this document is to introduce two SIEVE tests that can be used to implement 'generic' tests for spam and virii in messages processed via SIEVE scripts. These tests return a string containing a range of numeric values that indicate the severity of spam or virii in a message, or a string that indicates the message has not passed through any spam or virus checking tools. The spam and virus checks themselves are handled by the underlying SIEVE implementation in whatever manner is appropriate, and the implementation maps the results of these checks into the numeric ranges defined by the new tests. Thus a SIEVE implementation can have a spam test that implicitly checks for third-party spam tool headers and determines how those map into the spamtest numeric range. In order to do numeric comparisons against the returned strings, server implementations MUST also support the SIEVE relational [RELATIONAL] extension, in addition to the extensions described here. All examples below assume the relational extension is present. 5 SIEVE Extensions 5.1 spamtest Syntax: spamtest [COMPARATOR] [MATCH-TYPE] SIEVE implementations that implement the "spamtest" test have an identifier of "spamtest" for use with the capability mechanism. The "spamtest" test evaluates to true if the spamtest result matches the value. The type of match is specified by the optional match argument, which defaults to ":is" if not specified. The spamtest result is a string containing a numeric value in the range "0" (zero) through "10", with "0" meaning the message is Daboo Expires September 2003 [Page 3] Internet Draft SIEVE spamtest and virustest Extensions March 2003 definitely clear of spam, and "10" meaning the message is definitely spam. The underlying SIEVE implementation will map whatever spam check is done into this numeric range, as appropriate. If the message has not been categorised by any spam checking tools, then the spamtest result is "NIL". Examples: require ["spamtest", "fileinto", "relational", "comparator-i;ascii-numeric"]; if spamtest :value "ge" :comparator "i;ascii-numeric" "3" { fileinto "INBOX.spam-trap"; } elsif spamtest :is "NIL" { fileinto "INBOX.unclassified"; } In this example, any message with a spamtest value greater than or equal to "3" is filed into a mailbox called "INBOX.spam-trap" in the user's mailstore. Any message that has not passed through a spam check tool will be filed into the mailbox "INBOX.unclassified". 5.2 virustest Syntax: virustest [COMPARATOR] [MATCH-TYPE] SIEVE implementations that implement the "virustest" test have an identifier of "virustest" for use with the capability mechanism. The "virustest" test evaluates to true if the virustest result matches the value. The type of match is specified by the optional match argument, which defaults to ":is" if not specified. The virustest result is a string containing a numeric value in the range "0" (zero) through "2", with "0" meaning the message is definitely clear of virii, "1" meaning the message may contain virii (looks suspicious) and "2" meaning the message definitely contains a virus. The underlying SIEVE implementation will map whatever virus check is done into this numeric range, as appropriate. If the message has not been categorised by any virus checking tools, then the virustest result is "NIL". Daboo Expires September 2003 [Page 4] Internet Draft SIEVE spamtest and virustest Extensions March 2003 Example: require ["virustest", "fileinto", "relational", "comparator-i;ascii-numeric"]; if virustest :value "eq" :comparator "i;ascii-numeric" "1" { fileinto "INBOX.quarantine"; } elsif virustest :value "eq" :comparator "i;ascii-numeric" "2" { discard; } elsif virustest :is "NIL" { fileinto "INBOX.unclassified"; } In this example, any message with a virustest value equal to "1" is filed into a mailbox called "INBOX.quarantine" in the user's mailstore. Any message with a virustest value equal to "2" is discarded (removed) and not delivered to the user's mailstore. Any message that has not passed through a virus check tool will be filed into the mailbox "INBOX.unclassified". 6 Security Considerations SIEVE implementations SHOULD ensure that "spamtest" and "virustest" tests can only occur for messages that have gone through a legitimate spam or virus check process. If such checks rely on the addition of special headers to messages, it is the responsibility of the implementation to ensure that such headers cannot be spoofed by the sender, to prevent the implementation from being tricked into returning the wrong result for the test. Beyond that, the "spamtest" and "virustest" extensions do not raise any security considerations that are not present in the base [SIEVE] protocol, and these issues are discussed in [SIEVE]. 7 Normative References [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RELATIONAL] Segmuller, W. "Sieve Extension: Relational Tests" draft-segmuller-sieve-relation-02.txt, May 2002. [SIEVE] Showalter, "Sieve: A Mail Filtering Language", RFC 3028, January 2001. Daboo Expires September 2003 [Page 5] Internet Draft SIEVE spamtest and virustest Extensions March 2003 8 Acknowledgments Thanks to Tony Hansen, Jutta Degener and Ned Freed for comments on the original draft. Thanks to Ashish Gawarikar for catching some example syntax errors. 9 Author's Address Cyrus Daboo Cyrusoft International, Inc. Suite 780, 5001 Baum Blvd. Pittsburgh, PA 15213 U.S.A. Email: daboo@cyrusoft.com 10 Full Copyright Statement Copyright (C) The Internet Society 2003. 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. Daboo Expires September 2003 [Page 6]