HTML sanitization: Difference between revisions

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

{{More 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]] (XSS) by sanitizing any HTML code submitted by a user.

== Details ==

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 An out-of-date blacklist can therefore be dangerous if new, unsafe features have been introduced to the HTML Standard.

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In [[Java (programming language)|Java]] (and [[.NET Framework|.NET]]), sanitization 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/ {{Dead|title=HTML Agility Pack - Home link|access-date=March2013-01-04 |archive-date=2013-01-01 |archive-url=https://web.archive.org/web/20130101170916/http://htmlagilitypack.codeplex.com/ |url-status=dead 2022}}</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).