Sieve spamtest: Sieve Email Filtering: Spamtest and Virustest Extensions

Network Working Group                                           C. Daboo
Request for Comments: 5235                                  January 2008
Obsoletes: 3685
Category: Standards Track


        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.

Abstract

   The Sieve email filtering language "spamtest", "spamtestplus", 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 proper input to the tests.

Table of Contents

   1. Introduction and Overview .......................................2
   2. Conventions Used in This Document ...............................2
   3. Sieve Extensions ................................................3
      3.1. General Considerations .....................................3
      3.2. Test spamtest ..............................................3
           3.2.1. spamtest without :percent Argument ..................4
           3.2.2. spamtest with :percent Argument .....................5
      3.3. Test virustest .............................................7
   4. Security Considerations .........................................9
   5. IANA Considerations .............................................9
      5.1. spamtest Registration ......................................9
      5.2. virustest Registration ....................................10
      5.3. spamtestplus Registration .................................10
   6. References .....................................................10
      6.1. Normative References ......................................10
      6.2. Informative References ....................................11
   Appendix A. Acknowledgments .......................................12
   Appendix B. Important Changes since RFC 3685 ......................12

1.  Introduction and Overview

   Sieve scripts are frequently being used to do spam and virus
   filtering either based on 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.  The spam and virus checks themselves
   are handled by the underlying Sieve implementation in whatever manner
   is appropriate, so that the Sieve spam and virus test commands can be
   used in a portable way.

   In order to do numeric comparisons against the returned strings,
   server implementations MUST also support the Sieve relational
   [RFC5231] extension, in addition to the extensions described here.
   All examples below assume the relational extension is present.

2.  Conventions Used in This Document

   Conventions for notations are as in [RFC5228] Section 1.1.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   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.  Sieve Extensions

3.1.  General Considerations

   The "spamtest" and "virustest" tests described below evaluate the
   results of implementation-specific spam and virus checks in a
   portable way.  The implementation may, for example, check for third-
   party spam tool headers and determine how those map into the way the
   test commands are used.  To do this, the underlying Sieve
   implementation provides a normalized result string as one of the
   inputs to each test command.  The normalized result string is
   considered to be the value on the left-hand side of the test, and the
   comparison values given in the test command are considered to be on
   the right-hand side.

   The normalized result starts with a digit string, with its numeric
   value within the range of values used by the specific test,
   indicating the severity of spam or viruses in a message or whether
   any tests were done at all.  This may optionally be followed by a
   space (%x20) character and arbitrary text, or in one specific case a
   single keyword is returned.  The numeric value can be compared to
   specific values using the Sieve relational [RFC5231] extension in
   conjunction with the "i;ascii-numeric" comparator [RFC4790], which
   will test for the presence of a numeric value at the start of the
   string, ignoring any additional text in the string.  The optional
   text can be used to carry implementation-specific details about the
   tests 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.

   In addition, the Sieve relational [RFC5231] ":count" match type can
   be used to determine if the underlying implementation actually did a
   test.  If the underlying spam or virus test was done, the ":count" of
   the normalized result will return the numeric value "1", whilst if
   the test was not done, or the Sieve implementation could not
   determine if a test was done or not done, the ":count" value will be
   "0" (zero).

3.2.  Test spamtest

           Usage:    spamtest [":percent"] [COMPARATOR] [MATCH-TYPE]
                     <value: string>

   Sieve implementations that implement the "spamtest" test use an
   identifier of either "spamtest" or "spamtestplus" for use with the
   capability mechanism.

   If the ":percent" argument is not used with any spamtest test, then
   one or both of "spamtest" or "spamtestplus" capability identifiers
   MUST be present.

   If the ":percent" argument is used with any spamtest test, then the
   "spamtestplus" capability identifier MUST be present.  Sieve
   implementations MUST return an error if the ":percent" argument is
   used and "spamtestplus" is not specified.

   In the interests of brevity and clarity, scripts SHOULD NOT specify
   both "spamtestplus" and "spamtest" capability identifiers together.

   The "spamtest" test evaluates to true if the normalized spamtest
   result matches the value.  The type of match is specified by the
   optional match argument, which defaults to ":is" if not specified.

3.2.1.  spamtest without :percent Argument

   When the ":percent" argument is not present in the "spamtest" test,
   the normalized result string provided for the left-hand side of the
   test starts 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, or Sieve could not    |
   |          | determine whether any test was done                    |
   |          |                                                        |
   | 1        | message was tested and is clear of spam                |
   |          |                                                        |
   | 2 - 9    | message was tested and may contain spam; a higher      |
   |          | number indicates a greater likelihood of spam          |
   |          |                                                        |
   | 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 normalized result value greater than or equal to "3"
   is filed into a mailbox called "INBOX.spam-trap" in the user's
   mailstore.

3.2.2.  spamtest with :percent Argument

   When the ":percent" argument is present in the "spamtest" test, the
   normalized result string provided for the left-hand side of the test
   starts with a numeric value in the range "0" (zero) through "100",
   with meanings summarized below:

   +----------+-------------------------------------------------------+
   | spamtest | interpretation                                        |
   | value    |                                                       |
   +----------+-------------------------------------------------------+
   | 0        | message was tested and is clear of spam, or was not   |
   |          | tested for spam, or Sieve could not determine whether |
   |          | any test was done                                     |
   |          |                                                       |
   | 1 - 99   | message was tested and may contain spam; a higher     |
   |          | percentage indicates a greater likelihood of spam     |
   |          |                                                       |
   | 100      | message was tested and definitely contains spam       |
   +----------+-------------------------------------------------------+

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

   To determine whether or not the message was tested for spam, two
   options can be used:

   a.  a test with or without the ":percent" argument and ":count" match
       type, testing for the value "0" as described in Section 3.1.

   b.  a test without the ":percent" argument using the ":value" match
       type, testing for the normalized result value "0" as described in
       Section 3.2.1.

   Examples:

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

           if spamtest :value "eq"
                       :comparator "i;ascii-numeric" "0"
           {
               fileinto "INBOX.unclassified";
           }
           elsif spamtest :percent :value "eq"
                          :comparator "i;ascii-numeric" "0"
           {
               fileinto "INBOX.not-spam";
           }
           elsif spamtest :percent :value "lt"
                          :comparator "i;ascii-numeric" "37"
           {
               fileinto "INBOX.spam-trap";
           }
           else
           {
               discard;
           }

   In this example, any message that has not passed through a spam check
   tool will be filed into the mailbox "INBOX.unclassified".  Any
   message that is classified as definitely not containing spam
   (normalized result value "0") will be filed into the mailbox
   "INBOX.not-spam".  Any message with a normalized result value less
   than "37" is filed into a mailbox called "INBOX.spam-trap" in the
   user's mailstore.  Any other normalized result value will result in
   the message being discarded.

   Alternatively, the Sieve relational [RFC5231] ":count" match type can
   be used:

   Examples:

           if spamtest :percent :count "eq"
                       :comparator "i;ascii-numeric" "0"
           {
               fileinto "INBOX.unclassified";
           }
           elsif spamtest :percent :value "eq"
                          :comparator "i;ascii-numeric" "0"
           {
               fileinto "INBOX.not-spam";
           }
           elsif spamtest :percent :value "lt"
                          :comparator "i;ascii-numeric" "37"
           {
               fileinto "INBOX.spam-trap";
           }
           else
           {
               discard;
           }

   This example will result in exactly the same behavior as the previous
   one.

3.3.  Test virustest

           Usage:    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 normalized result
   string matches the value.  The type of match is specified by the
   optional match argument, which defaults to ":is" if not specified.

   The normalized result string provided for the left side of the test
   starts 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, or Sieve could    |
   |           | not determine whether any test was done               |
   |           |                                                       |
   | 1         | message was tested and contains no known viruses      |
   |           |                                                       |
   | 2         | message was tested and contained a known virus that   |
   |           | was replaced with harmless content                    |
   |           |                                                       |
   | 3         | message was tested and contained a known virus that   |
   |           | 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 normalized result value equal to "4" is filed into a
   mailbox called "INBOX.quarantine" in the user's mailstore.  Any
   message with a normalized result value equal to "5" is discarded
   (removed) and not delivered to the user's mailstore.

4.  Security Considerations

   Sieve implementations SHOULD ensure that "spamtest" and "virustest"
   tests only report spam and virus test results for messages that
   actually have gone through a legitimate spam or virus check process.
   In particular, if such checks rely on the addition and subsequent
   checking of private header fields, it is the responsibility of the
   implementation to ensure that such headers cannot be spoofed by the
   sender or intermediary and thereby 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
   [RFC5228] protocol, and these issues are discussed in [RFC5228].

5.  IANA Considerations

   The following templates specify the IANA registration of the Sieve
   extensions specified in this document.  The registrations for
   "spamtest" and "virustest" replace those from [RFC3685]:

5.1.  spamtest Registration

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

      Capability name: spamtest
      Description:     Provides a test to check for varying likelihood of
                       an email message being spam.
      RFC number:      RFC 5235
      Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>

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

5.2.  virustest Registration

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

      Capability name: virustest
      Description:     Provides a test to check for varying likelihood of
                       there being malicious content in an email message.
      RFC number:      RFC 5235
      Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>

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

5.3.  spamtestplus Registration

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

      Capability name: spamtestplus
      Description:     Provides a test to check for varying likelihood of
                       an email message being spam, possibly using a
                       percentage range.
      RFC number:      RFC 5235
      Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>

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

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
              Application Protocol Collation Registry", RFC 4790, March
              2007.

   [RFC5228]  Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An Email
              Filtering Language", RFC 5228, January 2008.

   [RFC5231]  Segmuller, W. and B. Leiba, "Sieve Email Filtering:
              Relational Extension", RFC 5231, January 2008.

6.2.  Informative References

   [RFC3685]  Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest
              Extensions", RFC 3685, February 2004.

Appendix A.  Acknowledgments

   Thanks to Mark E. Mallett, Tony Hansen, Jutta Degener, Ned Freed,
   Ashish Gawarikar, Alexey Melnikov, Nigel Swinson, and Aaron Stone for
   comments and corrections.

Appendix B.  Important Changes since RFC 3685

   Listed below are some of the major changes from the previous
   specification [RFC3685], which this one supersedes.

   1. A ":percent" argument has been added to the "spamtest" test adding
      a new 0-100 numerical range for test results.

   2. A "spamtestplus" requires item has been added to indicate the
      presence of this extension in scripts.

   3. The "count" match type from [RFC5231] can now be used to determine
      whether or not a message was tested.

   4. Clarified that "test not done" also means "Sieve system could not
      determine if a test was done".

Author's Address

   Cyrus Daboo

   EMail: cyrus@daboo.name

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