Abridged XSS Prevention Cheat Sheet

Introduction

Cross site scripting is the most common web vulnerability. Cross Site Scripting is a dangerous threat since it allows an attacker to trick a victim into executing malicious client-side script in a browser. This cheat sheet is a derivative work of the XSS (Cross Site Scripting) Prevention Cheat Sheet and will assist web developers in eliminating XSS from their applications.

XSS Prevention Safe Contexts

The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts.

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 Only place untrusted data into a whitelist of safe attributes (listed below). 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 in a SRC or HREF attribute	<a href="UNTRUSTED URL">clickme</a> <iframe src="UNTRUSTED URL" />	Cannonicalize input URL Validation Safe URL verification Whitelist http and https URL's only (Avoid the JavaScript Protocol to Open a new Window) Attribute encoder
String	CSS Value	<div style="width: UNTRUSTED DATA;">Selection</div>	Strict structural validation CSS Hex encoding Good design of CSS Features
String	JavaScript Variable	<script>var currentValue='UNTRUSTED DATA';</script> <script>someFunction('UNTRUSTED DATA');</script>	Ensure JavaScript variables are quoted JavaScript Hex Encoding JavaScript Unicode Encoding Avoid backslash encoding (\" or \' or \\)
HTML	HTML Body	<div>UNTRUSTED HTML</div>	HTML Validation (JSoup, AntiSamy, HTML Sanitizer)
JavaScript	HTML Body	<div>UNTRUSTED JAVASCRIPT</div>	Sandboxing with tools such as JSReg
String	DOM XSS	TODO	DOM based XSS Prevention Cheat Sheet
String	AJAX/JSON Parsing	JSON.parse(UNTRUSTED JSON DATA)	Use JSON.parse or json2.js library to parse JSON Avoid parsing JSON with eval() as it will execute any script included with the JSON
String	AJAX/XML Parsing	TODO	TODO
String	Framework Protections	<span>UNTRUSTED DATA</span>	Content Security Policy HTML5 Frame Sandbox
String	Loading HTML fragments using AJAX	TODO	When building HTML fragments from trusted HTML and untrusted data, adhere to the rules for HTML body, attributes, css, javascripts and URLs as stated above

Safe HTML Attributes 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

XSS Prevention Dangerous Contexts

The following snippets of HTML demonstrate dangerous contexts that developers should always avoid.

Data Type	Context	Code Sample	Danger
String	JavaScript eval(), setTimeout(), setInterval()	<script>eval(UNTRUSTED DATA);<script>	Untrusted data will still execute in JavaScript code execution contexts, even when encoded. Consider using a JavaScript Closure Within Eval.
String	HTML Comment	<!-- UNTRUSTED DATA -->	There are a variety of browser quirks that make this context dangerous, even when encoded.
String	JavaScript Comment	/* UNTRUSTED DATA */	Danger, for example Chrome JavaScript comments [1]

How to Output Encode

The purpose of output encoding (as it relates to Cross Site Scripting) is to convert untrusted input into a safe form where the input is displayed as data to the user without executing as code in the browser. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting.

Encoding Type	Encoding Mechanism
HTML Entity Encoding	Convert & to &amp; Convert < to &lt; Convert > to &gt; Convert " to &quot; Convert ' to &#x27; Convert / to &#x2F;
HTML Attribute Encoding	Except for alphanumeric characters, escape all characters with the HTML Entity &#xHH; format, including spaces. (HH = Hex Value)
URL Encoding	Standard percent encoding, see: http://www.w3schools.com/tags/ref_urlencode.asp
JavaScript Encoding	Except for alphanumeric characters, escape all characters with the \uXXXX unicode escaping format (X = Integer).
CSS Hex Encoding	CSS escaping supports \XX and \XXXXXX. Using a two character escape can cause problems if the next character continues the escape sequence. There are two solutions (a) Add a space after the CSS escape (will be ignored by the CSS parser) (b) use the full amount of CSS escaping possible by zero padding the value.

Experimental Minimal Encoding Rules

The following examples demonstrate minimal experiential encoding rules.

Context	Code Sample	Rules
JavaScript, quoted string in a script block	alert("Hello "+"<%= UNTRUSTED DATA %>")	Use these escapes: \\ \r \n \b \t \f \' \" \/ For any other character in range 0..0x19, use hex escapes If using non-Unicode charset, any character above 0x7e, use '\u' encoding
JavaScript, quoted string in an event handler attribute	onclick="alert('<%= UNTRUSTED DATA %>')";	Use these escapes: \\ \r \n \b \t \f Use hex escapes for these characters: ' " & For any other character in range 0..0x19, use hex escapes If using non-Unicode charset, any character above 0x7e, use '\u' encoding
HTML Body (up to HTML 4.01):	<div><%= UNTRUSTED DATA %></div>	HTML Entity encode < & specify charset in metatag to avoid UTF7 XSS
XHTML Body:	<div><%= UNTRUSTED DATA %></div>	HTML Entity encode < & > limit input to charset http://www.w3.org/TR/2008/REC-xml-20081126/#charsets

Related Articles

OWASP Cheat Sheets Project Homepage

• OWASP Cheat Sheet Series

Authors and Primary Editors

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