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Difference between revisions of "Abridged XSS Prevention Cheat Sheet"

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= DRAFT CHEAT SHEET - WORK IN PROGRESS =
 
 
= Introduction =
 
= Introduction =
  

Revision as of 02:15, 20 November 2011

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. This cheat sheet is a derivative work of the XSS (Cross Site Scripting) Prevention Cheat Sheet.

XSS Prevention

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.
  • 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 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
*/
  • Danger, for example Chrome JavaScript comments [1]
HTML HTML Body <div>UNTRUSTED HTML</div>
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/HTML TODO
  • Userdata must be properly encoded serverside according to the rules above
String AJAX/XML Parsing TODO TODO

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

Output Encoding

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.

Related Articles

OWASP Cheat Sheets Project Homepage


V - T - E Cheat Sheets
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Assessment / Breaker
Mobile
OpSec / Defender
Draft and Beta
All Pages In This Category

Authors and Primary Editors

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

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