Abridged XSS Prevention Cheat Sheet

Introduction

Cross site scripting is the most common web vulnerability. It represents a serious threat because cross site scripting allows evil attacker code to run in a victim’s browser. More details about XSS can be found here: https://www.owasp.org/index.php/Cross-site_Scripting_%28XSS%29

XSS Prevention Overview

Data Type	Context	Code Sample	Defense
String	HTML Body	<span>UNTRUSTED DATA</span>	HTML Entity Encoding
String	"safe" HTML Attributes	<input type="text" name="fname" value="UNTRUSTED DATA">	Aggressive HTML Entity Encoding Avoid placing untrusted data in an ID attribute (it can influence the DOM even when escaped) Only place untrusted data into a whitelist of safe attributes white include: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width Strictly validate unsafe attributes such as background, id and name.
String	GET Parameter	<a href="/site/search?value=UNTRUSTED DATA">clickme</a>	URL Encoding
String	Untrusted URL rendered in an HREF tag (or other HTML link context)	<a href="UNTRUSTED DATA">clickme</a> <iframe src="UNTRUSTED DATA" />	Cannonicalize input URL Validation Safe URL verification Whitelist http and https URL's only Attribute encoder
String	CSS	<div style="width: UNTRUSTED DATA;">Selection</div>	Strict structural validation CSS Hex encoding good design of CSS Features
String	JavaScript	<script>var currentValue='UNTRUSTED DATA';</script>	Ensure JavaScript variables are quoted JavaScript Hex Encoding JavaScript Unicode Encoding Avoid backslash encoding (\" or \' or \\)
String	HTML Comment	<!-- UNTRUSTED DATA-->	TODO
String	JavaScript Comment	/* UNTRUSTED DATA */	TODO
HTML Text	HTML Body	<span>UNTRUSTED HTML</span>	HTML Validation (JSoup, AntiSamy, HTML Sanitizer)
String	DOM XSS	TODO	DOM based XSS Prevention Cheat Sheet
String	AJAX/JSON Parsing	TODO	Use JSON.parse or json2.js library to parse JSON Avoid parsing JSON with eval()
String	AJAX/XML Parsing	TODO	TODO

Output Encoding Types

Encoding Type	Encoding Mechanism
HTML Entity Encoding	& --> &amp; < --> &lt; > --> &gt; " --> &quot; ' --> &#x27; ' is not recommended / --> &#x2F; forward slash is included as it helps end an HTML entity
HTML Attribute Encoding	TODO
URL Encoding	TODO
JavaScript HEX Encoding	TODO
CSS Hex Encoding	TODO

Related Articles

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Authors and Primary Editors

Jim Manico - jim [at] owasp.org
Jeff Williams - jeff [at] aspectsecurity.com