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Difference between revisions of "Abridged XSS Prevention Cheat Sheet"

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Revision as of 08:00, 1 May 2012

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>
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>
String Untrusted URL in a SRC or HREF attribute <a href="UNTRUSTED URL">clickme</a>
<iframe src="UNTRUSTED URL" />
String CSS Value <div style="width: UNTRUSTED DATA;">Selection</div>
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>
JavaScript HTML Body <div>UNTRUSTED JAVASCRIPT</div>
  • Sandboxing with tools such as JSReg
String DOM XSS TODO
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>
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>
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>

Related Articles

OWASP Cheat Sheets Project Homepage


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OpSec / Defender
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Authors and Primary Editors

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

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