HTML sanitization: Difference between revisions

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{{Short description|Process of removing undesirable parts of an HTML document}}

{{Orphan|date=December 2009}}

{{RefimproveMore citations needed|date=December 2009}}

In [[data sanitization]], '''HTML sanitization''' is the process of examining an [[HTML]] document and producing a new HTML document that preserves only whatever tags and attributes are designated "safe" and desired. HTML sanitization can be used to protect against attacks such as [[cross-site scripting]] and [[SQL injection]] attacks(XSS) by sanitizing any HTML code submitted by a user.

== Details ==

Tags often allowed are <nowiki><b></nowiki>, <nowiki><i></nowiki>, <nowiki><u></nowiki>, <nowiki><em></nowiki>, and <nowiki><strong></nowiki>.

Basic tags for changing fonts are often allowed, such as <code>&lt;b&gt;</code>, <code>&lt;i&gt;</code>, <code>&lt;u&gt;</code>, <code>&lt;em&gt;</code>, and <code>&lt;strong&gt;</code> while more advanced tags such as <code>&lt;script&gt;</code>, <code>&lt;object&gt;</code>, <code>&lt;embed&gt;</code>, and <code>&lt;link&gt;</code> are removed by the sanitization process. Also potentially dangerous [[HTML attribute|attributes]] such as the <code>onclick</code> attribute are removed in order to prevent malicious code from being injected.

Sanitization is typically performed by using either a [[whitelist]] or a [[Blacklist (computing)|blacklist]] approach. Leaving a safe HTML element off a whitelist is not so serious; it simply means that that feature will not be included post-sanitation. On the other hand, if an unsafe element is left off a blacklist, then the vulnerability will not be sanitized out of the HTML output. An out-of-date blacklist can therefore be dangerous if new, unsafe features have been introduced to the HTML Standard.

In [[PHP]] this can be performed using the <code>strip_tags()</code> function.<ref>http://www.php.net/strip_tags</ref>

Further sanitization can be performed based on rules which specify what operation is to be performed on the subject tags. Typical operations include removal of the tag itself while preserving the content, preserving only the textual content of a tag or forcing certain values on attributes.<ref name="HtmlRuleSanitizer">{{Cite web|url=https://github.com/Vereyon/HtmlRuleSanitizer|title = HtmlRuleSanitizer|website = [[GitHub]]|date = 13 August 2021}}</ref>

In [[Java]] this can be achieved by using OWASP Java HTML Sanitizer Project <ref>https://www.owasp.org/index.php/OWASP_Java_HTML_Sanitizer_Project</ref>

== Implementations ==

In [[PHP]], HTML sanitization can be performed using the <code>strip_tags()</code> function at the risk of removing all textual content following an unclosed less-than symbol or angle bracket.<ref>{{cite web|url=http://us3.php.net/manual/en/function.strip-tags.php|title=strip_tags|publisher=PHP.NET}}</ref> The HTML Purifier library is another popular option for PHP applications.<ref>{{Cite web|url=http://htmlpurifier.org/|title=HTML Purifier - Filter your HTML the standards-compliant way!|website=htmlpurifier.org}}</ref>

In [[Java (programming language)|Java]] (and [[.NET Framework|.NET]]), thissanitization can be achieved by using the [[OWASP]] Java HTML Sanitizer Project .<ref>{{Cite web|url=https://www.owasp.org/index.php/OWASP_Java_HTML_Sanitizer_Project|title = OWASP Java HTML Sanitizer}}</ref>

In [[.NET Framework|.NET]], a number of sanitizers use the Html Agility Pack, an HTML parser.<ref>{{Cite web |url=http://htmlagilitypack.codeplex.com/ |title=HTML Agility Pack - Home |access-date=2013-01-04 |archive-date=2013-01-01 |archive-url=https://web.archive.org/web/20130101170916/http://htmlagilitypack.codeplex.com/ |url-status=dead }}</ref><ref>{{Cite web|url=http://eksith.wordpress.com/2011/06/14/whitelist-santize-htmlagilitypack/|title = Whitelist santize with HtmlAgilityPack|date = 14 June 2011}}</ref><ref name="HtmlRuleSanitizer" /> Another library is HtmlSanitizer.<ref>{{cite web |last1=Ganss |first1=Michael |title=HtmlSanitizer |url=https://github.com/mganss/HtmlSanitizer/ |access-date=7 December 2023 |date=5 December 2023}}</ref>

In [[JavaScript]] there are "JS-only" sanitizers for the [[front and back ends|back end]], and browser-based<ref>{{Cite web|url=https://github.com/jitbit/HtmlSanitizer|title=JS HTML Sanitizer|website=[[GitHub]]|date=14 October 2021}}</ref> implementations that use browser's own [[Document Object Model]] (DOM) parser to parse the HTML (for better performance).

== References ==

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[[Category:HTML]]

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