RFC 3685 PROPOSED STANDARD EMAIL · FILTERING · SPAM
Sieve spamtest: SIEVE Email Filtering: Spamtest and VirusTest Extensions
Portable spam and virus scoring tests for email filters.
RFC EDITOR
Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest Extensions", RFC 3685, DOI 10.17487/RFC3685, February 2004, <https://www.rfc-editor.org/info/rfc3685>.
BIBTEX
@misc{rfc3685,
series = {Request for Comments},
number = 3685,
howpublished = {RFC 3685},
publisher = {RFC Editor},
doi = {10.17487/RFC3685},
url = {https://www.rfc-editor.org/info/rfc3685},
author = {C. Daboo},
title = {{SIEVE Email Filtering: Spamtest and VirusTest Extensions}},
pagetotal = 9,
year = 2004,
month = feb,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
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.
AI ELI5
On this page
PROBLEMEvery spam and virus scanning tool uses its own format to report results, making email filter scripts fragile and specific to particular tools.
MECHANISMThe spamtest and virustest extensions provide standardized numeric scores (0-10 for spam, 0-5 for viruses) that filter implementations map from whatever tools they use underneath.
WHY IT MATTERSEmail users can write portable Sieve filters that work across different server implementations without depending on the specific scanner tool. RFC 5235 replaced it with an updated version.
§1Introduction and Overview
The SIEVE language is used by email servers to let users write rules that filter their mail. Before these extensions, many organizations used SIEVE with external spam and virus scanning tools, but every scanner tool reported results differently: some added headers with special syntax, some used codes only they understood. Users had to write custom filters for each tool, and when the tool changed or the server switched to a different scanner, the filters would break.
The spamtest and virustest extensions solve this by letting the server itself expose standardized numeric scores. The Sieve implementation can check mail with whatever scanners it uses, then report a simple number: for spam, a value from 0 to 10; for viruses, a value from 0 to 5. Users write rules that compare these numbers, and their scripts work on any compliant server.
Email servers use tools to check for spam and viruses, but each tool reports its findings differently. These extensions give a standard way to ask the server for a number from 0 to 10 for spam or 0 to 5 for viruses.
§2How the Tests Work
Both tests return a string that starts with a numeric value, optionally followed by a space and additional text. The string is compared using the SIEVE relational extension with the i;ascii-numeric comparator, which extracts just the numeric part at the beginning and ignores the rest. This lets implementations include extra details or reasons for the score without breaking the user's script.
Comparisons use SIEVE's standard match types like :is (equal), :ge (greater than or equal), and :lt (less than). The examples in the RFC use the relational extension to compare spam or virus scores numerically, allowing users to write rules like if spamtest :value "ge" "3" { fileinto "spam"; }.
The server reports a number: the higher the number, the more likely the mail is spam or infected. Your filter can say, if the number is 5 or higher, move it to the spam folder.
§3Spamtest Scores
The spamtest test evaluates to true when the spam score matches a value using the specified comparison. The score is a numeric value from 0 to 10 with the following meanings:
0: message was not tested for spam (no scanner ran on it)1: message was tested and is clear of spam2-9: message was tested and has varying likelihood of containing spam, with higher numbers indicating higher likelihood10: message was tested and definitely contains spam
The server implementation maps the actual results from its spam detection tools into this standard range. For example, if a spam detector returns a confidence percentage, the implementation might scale it to fit this 0-10 scale.
The spam score goes from 0 (definitely not spam) to 10 (definitely spam). The server figures out where to put mail from its own spam tools.
- 0Not tested — No spam scanner was run.
- 1Clean — Scanned and no spam detected.
- 2-9Likely spam — Increasing likelihood of spam.
- 10Definitely spam — Scanned and flagged as spam.
§4Virus Scores and Security
The virustest test works similarly, with a numeric value from 0 to 5:
0: message was not tested for viruses1: message was tested and contains no known viruses2: message was tested and contained a known virus which was replaced with harmless content3: message was tested and contained a known virus which was cured so it is now harmless4: message was tested and possibly contains a known virus5: message was tested and definitely contains a known virus
One critical security concern: if the implementation relies on headers added by scanners, it must ensure users cannot forge these headers to trick the filter into giving a false score. Administrators must also keep virus scanning tools up to date. However, the tests do not provide 100% virus protection, so users should still exercise caution with unknown senders.
Virus scores go from 0 (no viruses) to 5 (definitely infected). But remember: no scanner catches everything, so stay careful with suspicious emails.
§5Filtering with Scores
The RFC provides examples of how to use these tests. In one example, a filter sorts messages into different mailboxes based on their spam score: messages that were not scanned go to "INBOX.unclassified", and messages with a spamtest value of 3 or higher go to "INBOX.spam-trap". Another example quarantines messages with a virustest value of 4 and discards messages with a value of 5.
Because the scores are standard across implementations, users can move their filter scripts between different email servers without modification. The server is responsible for mapping its underlying tools to the standard numeric ranges, so the script remains portable.
You write a filter that says: if spam score is 3 or more, move to the spam folder. If virus score is 5, delete it. Your filter works the same on any email server.
§6References
[SIEVE]RFC 3028Showalter, T., "Sieve: A Mail Filtering Language", RFC 3028, January 2001.January 2001[RELATIONAL]RFC 3431Segmuller, W., "Sieve Extension: Relational Tests", RFC 3431, December 2002.December 2002[RFC5235]RFC 5235Daboo, C., "Sieve Email Filtering: Spamtest and VirusTest Extensions", RFC 5235, January 2008.January 2008[KEYWORDS]RFC 2119Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.March 1997[ACAP]RFC 2244Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997.November 1997