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Abridged XSS Prevention Cheat Sheet

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DRAFT CHEAT SHEET - WORK IN PROGRESS

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: <a href="https://www.owasp.org/index.php/Cross-site_Scripting_%28XSS%29">https://www.owasp.org/index.php/Cross-site_Scripting_%28XSS%29</a>

XSS Prevention Overview

Data Type Context Code Sample Defense
String HTML Body <span>UNTRUSTED DATA</span>
String "safe" HTML Attributes

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 		<input type="text" name="fname" value="UNTRUSTED DATA">
String GET Parameter <a href="/site/search?value=UNTRUSTED DATA">clickme</a>
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>
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>
String DOM XSS TODO
  • <a _fcknotitle="true" href="DOM based XSS Prevention Cheat Sheet">DOM based XSS Prevention Cheat Sheet</a>
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; &apos; 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

OWASP Cheat Sheets Project Homepage

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Authors and Primary Editors

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

<a _fcknotitle="true" href="Category:Cheatsheets">Cheatsheets</a>
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