Editing Apache SpamAssassin

{{Short description|Open-source e-mail spam filter}}
{{Use dmy dates|date=August 2023}}
{{More citations needed |date=May 2024}}
{{Infobox software
| name = Apache SpamAssassin
| logo = Apache SpamAssassin logo.svg
| logo size = 210px
| screenshot = Reco spam.png
| developer = [[Apache Software Foundation]]<ref>{{cite web|url=http://svn.apache.org/repos/asf/spamassassin/trunk/CREDITS|title=Project Management Committee|website=The Apache Software Foundation|year=2022|access-date=23 August 2023}}</ref>
| released = {{Start date and age|2001|04|20}}
| latest release version = {{wikidata|property|preferred|references|edit|Q1503674|P348|P548=Q2804309}}
| latest release date = {{start date and age|{{wikidata|qualifier|preferred|single|Q1503674|P348|P548=Q2804309|P577}}}}
| operating system = [[Cross-platform]]
| repo = {{URL|https://svn.apache.org/repos/asf/spamassassin/|SpamAssassin Repository}}
| programming language = {{#statements:P277|from=Q1503674}}
| genre = [[Spam filter]]
| license = [[Apache License 2.0]]
}}
'''Apache SpamAssassin''' is a [[computer program]] used for [[anti-spam techniques|e-mail spam filtering]]. It uses a variety of spam-detection techniques, including [[Domain Name System|DNS]] and [[fuzzy checksum]] techniques, [[Bayesian spam filtering|Bayesian filtering]], external programs, blacklists and online databases. It is released under the [[Apache License|Apache License 2.0]] and is a part of the [[Apache Foundation]] since 2004.

The program can be integrated with the [[Mail transfer agent|mail server]] to automatically filter all mail for a site. It can also be run by individual users on their own mailbox and integrates with several [[mail user agent|mail programs]]. Apache SpamAssassin is highly configurable; if used as a system-wide filter it can still be configured to support per-user preferences.

==History==
Apache SpamAssassin was created by Justin Mason, who had maintained a number of patches against an earlier program named ''filter.plx'' by Mark Jeftovic, which in turn was begun in August 1997. Mason rewrote all of Jeftovic's code from scratch and uploaded the resulting codebase to [[SourceForge]] on April 20, 2001.<ref>{{cite web |title=SpamAssassin Prehistory |url=https://spamassassin.apache.org/old/prehistory/index.html |publisher=Apache Foundation |access-date=19 December 2018}}</ref>

In Summer 2004 the project became an [[Apache Software Foundation]] project and later officially renamed to ''Apache SpamAssassin''.<ref>{{cite web |title=SpamAssassin Project Incubation Status |url=http://incubator.apache.org/projects/spamassassin.html |publisher=Apache Foundation |access-date=19 December 2018}}</ref>

==Methods of usage==
Apache SpamAssassin is a [[Perl]]-based application ({{mono|Mail::SpamAssassin}} in [[CPAN]]) which is usually used to filter all incoming mail for one or several users. It can be run as a [[Computer process|standalone application]] or as a subprogram of another application (such as a [[Milter]], [[SA-Exim]], [[Exiscan]], [[MailScanner]], [[MIMEDefang]], [[Amavis]]) or as a [[client (computing)|client]] ({{mono|spamc}}) that communicates with a [[daemon (computer software)|daemon]] ({{mono|spamd}}). The client/server or embedded mode of operation has performance benefits, but under certain circumstances may introduce additional security risks.

Typically either variant of the application is set up in a generic [[mail filter]] program, or it is called directly from a [[mail user agent]] that supports this, whenever new mail arrives. Mail filter programs such as [[procmail]] can be made to [[pipe (computing)|pipe]] all incoming mail through Apache SpamAssassin with an adjustment to a user's {{mono|procmailrc}} file.

==Operation==
Apache SpamAssassin comes with a large set of rules which are applied to determine whether an email is spam or not. Most rules are based on [[regular expression]]s that are matched against the body or header fields of the message, but Apache SpamAssassin also employs a number of other spam-fighting techniques. The rules are called "tests" in the SpamAssassin documentation.

Each test has a score value that will be assigned to a message if it matches the test's criteria. The scores can be positive or negative, with positive values indicating "spam" and negative "ham" (non-spam messages). A message is matched against all tests and Apache SpamAssassin combines the results into a global score which is assigned to the message. The higher the score, the higher the probability that the message is spam.

Apache SpamAssassin has an internal (configurable) score threshold to classify a message as spam. Usually a message will only be considered as spam if it matches multiple criteria; matching just a single test will not usually be enough to reach the threshold.

If Apache SpamAssassin considers a message to be spam, it can be further rewritten. In the default configuration, the content of the mail is appended as a [[MIME]] attachment, with a brief excerpt in the message body, and a description of the tests which resulted in the mail being classified as spam. If the score is lower than the defined settings, by default the information about the tests passed and total score is still added to the email headers and can be used in post-processing for less severe actions, such as tagging the mail as suspicious.

Apache SpamAssassin allows for a per-user configuration of its behavior, even if installed as system-wide service; the configuration can be read from a file or a database.  In their configuration users can specify individuals whose emails are never considered spam, or change the scores for certain rules. The user can also define a list of languages which they want to receive mail in, and Apache SpamAssassin then assigns a higher score to all mails that appear to be written in another language.

Apache SpamAssassin is based on heuristics (pattern recognition), and such software exhibits false positives and false negatives.

==Network-based filtering methods==
Apache SpamAssassin also supports:
* [[DNSBL|DNS-based blacklists]] and [[DNSWL|DNS-based whitelists]]
* Fuzzy-checksum-based spam detection filters such as the [[Distributed Checksum Clearinghouse]], [http://razor.sourceforge.net/ Vipul's Razor] {{Webarchive|url=https://web.archive.org/web/20130328202325/http://razor.sourceforge.net/ |date=28 March 2013 }} and the Cloudmark Authority plugins (commercial)
* [[Hashcash]] email stamps based on [[Proof-of-work system|proof-of-work]]
* [[Sender Policy Framework]] and [[DomainKeys Identified Mail]]
* [[URI]] blacklists such as [[SURBL]] or [https://URIBL.com URIBL] which track spam websites

More methods can be added reasonably easily by writing a Perl plug-in for Apache SpamAssassin.

==Bayesian filtering==
Apache SpamAssassin reinforces its rules through [[Bayesian spam filtering|Bayesian filtering]] where a user or administrator "feeds" examples of good (ham) and bad (spam) into the filter in order to learn the difference between the two. For this purpose, Apache SpamAssassin provides the command-line tool {{mono|sa-learn}}, which can be instructed to learn a single mail or an entire mailbox as either ham or spam.

Typically, the user will move unrecognized spam to a separate folder, and then run {{mono|sa-learn}} on the folder of non-spam and on the folder of spam separately. Alternatively, if the mail user agent supports it, {{mono|sa-learn}} can be called for individual emails. Regardless of the method used to perform the learning, SpamAssassin's Bayesian test will help score future e-mails based on this learning to improve the accuracy.

==Licensing==
Apache SpamAssassin is [[free software|free]]/[[open source software]], licensed under the [[Apache License|Apache License 2.0]]. Versions prior to 3.0 are dual-licensed under the [[Artistic License]] and the [[GNU General Public License]].

Many commercially available anti-spam packages integrate SpamAssassin as part of their products, such as SpamKiller by [[McAfee]] and [[Kerio MailServer]] by Kerio.<ref name="Hong">{{cite book |last1=Hong |first1=Bryan |title=Building A Server with FreeBSD 7: A Modular Approach |date=2008 |publisher=No Starch Press |location=San Francisco |isbn=9781593271459 |page=197 |edition=1st}}</ref>

==sa-compile==
<code>sa-compile</code> is a utility distributed with Apache SpamAssassin that compiles a SpamAssassin ruleset into a [[deterministic finite automaton]] that allows Apache SpamAssassin to use processor power more efficiently.

==Testing==
Apache SpamAssassin is designed to trigger on the [[GTUBE]], a 68-byte string similar to the antivirus [[EICAR test file]]. If this string is inserted in an RFC 5322 formatted message and passed through the Apache SpamAssassin engine, Apache SpamAssassin will trigger with a weight of 1000.

==See also==
{{Portal|Free and open-source software}}
*[[Anti-spam techniques]]

==Notes==
{{Reflist}}

==References==
{{Refbegin}}
*{{cite book
| first1    = Alistair
| last1     = McDonald
| title     = SpamAssassin: A Practical Guide to Integration and Configuration
| publisher = [[Packt|Packt Publishing]]
| edition   = 1st
| page     = 240
| date      = September 27, 2004
| isbn      = 978-1-904811-12-1
| url       = http://www.packtpub.com/spamassassin/book
}}
*{{cite book |first1     = Alan
 |last1      = Schwartz
 |title      = SpamAssassin
 |publisher  = [[O'Reilly Media]]
 |edition    = 1st
 |page       = [https://archive.org/details/spamassassin00schw/page/207 207]
 |date       = July 2004
 |isbn       = 978-0-596-00707-2
 |url        = https://archive.org/details/spamassassin00schw/page/207
 |url-access = registration
}}
{{Refend}}

==External links==
* {{official website}}
* [https://cwiki.apache.org/confluence/display/SPAMASSASSIN/ Apache SpamAssassin Wiki]
* [https://cwiki.apache.org/confluence/display/SPAMASSASSIN/RuleUpdates Apache SpamAssassin Rule Updates Wiki] Automatically updating Apache SpamAssassin
* [https://mcgrail.com/template/projects#KAM1 KAM.cf] KAM Ruleset for Apache SpamAssassin

{{Apache Software Foundation}}
{{Perl}}

{{DEFAULTSORT:Spamassassin}}
[[Category:Apache Software Foundation projects|SpamAssassin]]
[[Category:Cross-platform software]]
[[Category:Free email software]]
[[Category:Free software programmed in Perl]]
[[Category:Anti-spam]]
[[Category:Spamming]]
[[Category:Email-related software for Linux]]
[[Category:2001 software]]