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RFC 3685 - SIEVE Email Filtering: Spamtest and VirusTest Extensi


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Network Working Group                                           C. Daboo
Request for Comments: 3685                  Cyrusoft International, Inc.
Category: Standards Track                                  February 2004

        SIEVE Email Filtering: Spamtest and VirusTest Extensions

Status 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 (2004).  All Rights Reserved.

Abstract

   The SIEVE mail filtering language "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.

Table of Contents

   1.  Introduction and Overview  . . . . . . . . . . . . . . . . . .  2
   2.  SIEVE Extensions . . . . . . . . . . . . . . . . . . . . . . .  3
       2.1.  General Considerations . . . . . . . . . . . . . . . . .  3
       2.2.  Test spamtest. . . . . . . . . . . . . . . . . . . . . .  3
       2.3.  Test virustest . . . . . . . . . . . . . . . . . . . . .  4
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
       4.1.  spamtest registration. . . . . . . . . . . . . . . . . .  6
       4.2.  virustest registration . . . . . . . . . . . . . . . . .  6
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       5.1.  Normative References . . . . . . . . . . . . . . . . . .  7
       5.2.  Informative References . . . . . . . . . . . . . . . . .  7
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Intellectual Property Rights Statement . . . . . . . . . . . .  7
   8.  Author's Address . . . . . . . . . . . . . . . . . . . . . . .  8
   9.  Full Copyright Statement . . . . . . . . . . . . . . . . . . .  9

1.  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
   synchronization 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 viruses in messages
   processed via SIEVE scripts.  These tests return a string containing
   a range of numeric values that indicate the severity of spam or
   viruses 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.

   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.

2.  SIEVE Extensions

2.1.  General Considerations

   The "spamtest" and "virustest" tests described below both return a
   string that starts with a numeric value, followed by an optional
   space (%x20) character and optional arbitrary text.  The numeric
   value can be compared to specific values using the SIEVE relational
   [RELATIONAL] extension in conjunction with the "i;ascii-numeric"
   comparator [ACAP], which will test for the presence of a numeric
   value at the start of the string, ignoring any additional text in the
   string.  The additional text can be used to carry implementation
   specific details about the tests performed and descriptive comments
   about the result.  Tests can be done using standard string
   comparators against this text if it helps to refine behavior, however
   this will break portability of the script as the text will likely be
   specific to a particular implementation.

2.2.  Test spamtest

      Syntax: spamtest [COMPARATOR] [MATCH-TYPE] <value: string>

   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 starting with a numeric value in the
   range "0" (zero) through "10", with meanings summarized below:

      spamtest    interpretation
      value

         0          message was not tested for spam
         1          message was tested and is clear of spam
         2 - 9      message was tested and has a varying likelihood of
                    containing spam in increasing order
         10         message was tested and definitely contains spam

   The underlying SIEVE implementation will map whatever spam check is
   done into this numeric range, as appropriate.

      Examples:

         require ["spamtest", "fileinto",
                  "relational", "comparator-i;ascii-numeric"];

         if spamtest :value "eq" :comparator "i;ascii-numeric" "0"
         {
             fileinto "INBOX.unclassified";
         }
         elsif spamtest :value "ge" :comparator "i;ascii-numeric" "3"
         {
             fileinto "INBOX.spam-trap";
         }

   In this example, any message that has not passed through a spam check
   tool will be filed into the mailbox "INBOX.unclassified".  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.

2.3.  Test virustest

      Syntax: virustest [COMPARATOR] [MATCH-TYPE] <value: string>

   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 starting with a numeric value in the
   range "0" (zero) through "5", with meanings summarized below:

      virustest   interpretation
      value

         0          message was not tested for viruses
         1          message was tested and contains no known viruses
         2          message was tested and contained a known virus which
                    was replaced with harmless content
         3          message was tested and contained a known virus
                    which was "cured" such that it is now harmless
         4          message was tested and possibly contains a
                    known virus
         5          message was tested and definitely contains a
                    known virus

   The underlying SIEVE implementation will map whatever virus checks
   are done into this numeric range, as appropriate.  If the message has
   not been categorized by any virus checking tools, then the virustest
   result is "0".

   Example:

      require ["virustest", "fileinto",
               "relational", "comparator-i;ascii-numeric"];

      if virustest :value "eq" :comparator "i;ascii-numeric" "0"
      {
          fileinto "INBOX.unclassified";
      }
      if virustest :value "eq" :comparator "i;ascii-numeric" "4"
      {
          fileinto "INBOX.quarantine";
      }
      elsif virustest :value "eq" :comparator "i;ascii-numeric" "5"
      {
          discard;
      }

   In this example, any message that has not passed through a virus
   check tool will be filed into the mailbox "INBOX.unclassified".  Any
   message with a virustest value equal to "4" is filed into a mailbox
   called "INBOX.quarantine" in the user's mailstore.  Any message with
   a virustest value equal to "5" is discarded (removed) and not
   delivered to the user's mailstore.

3.  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.

   Server administrators MUST ensure that the virus checking tools are
   kept up to date, to provide reasonable protection for users using the
   "virustest" test.  Users should be made aware of the fact that the
   "virustest" test does not provide a 100% reliable way to remove all
   viruses, and they should continue to exercise caution when dealing
   with messages of unknown content and origin.

   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].

4.  IANA Considerations

   The following templates specify the IANA registration of the Sieve
   extensions specified in this document:

4.1.  spamtest registration

   To: iana@iana.org
   Subject: Registration of new Sieve extension

   Capability name: spamtest
   Capability keyword: spamtest
   Capability arguments: N/A
   Standards Track/IESG-approved RFC XXXX: this RFC
   Person and email address to contact for further information:

      Cyrus Daboo
      Cyrusoft International, Inc.
      5001 Baum Blvd., Suite 780,
      Pittsburgh, PA 15213
      U.S.A.

      <mailto:daboo@cyrusoft.com>

   This information has been added to the list of sieve extensions given
   on http://www.iana.org/assignments/sieve-extensions.

4.2.  virustest registration

   To: iana@iana.org
   Subject: Registration of new Sieve extension

   Capability name: virustest
   Capability keyword: virustest
   Capability arguments: N/A
   Standards Track/IESG-approved RFC XXXX: this RFC
   Person and email address to contact for further information:

      Cyrus Daboo
      Cyrusoft International, Inc.
      5001 Baum Blvd., Suite 780,
      Pittsburgh, PA 15213
      U.S.A.

      <mailto:daboo@cyrusoft.com>

   This information has been added to the list of sieve extensions given
   on http://www.iana.org/assignments/sieve-extensions.

5.  References

5.1.  Normative References

   [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RELATIONAL] Segmuller, W., "Sieve Extension: Relational Tests", RFC
                3431, December 2002.

   [SIEVE]      Showalter, T., "Sieve: A Mail Filtering Language", RFC
                3028, January 2001.

5.2.  Informative References

   [ACAP]       Newman, C. and J. Myers, "ACAP -- Application
                Configuration Access Protocol", RFC 2244, November 1997.

6.  Acknowledgments

   Thanks to Tony Hansen, Jutta Degener, Ned Freed, Ashish Gawarikar and
   Nigel Swinson for comments and corrections.

7.  Intellectual Property Rights Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property 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; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication 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 implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

8.  Author's Address

   Cyrus Daboo
   Cyrusoft International, Inc.
   5001 Baum Blvd., Suite 780,
   Pittsburgh, PA 15213
   U.S.A.

   EMail: daboo@cyrusoft.com

9.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  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 assignees.

   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.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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