Introduction

This article provides a simple positive model for preventing XSS using output escaping/encoding properly. While there are a huge number of XSS attack vectors, following a few simple rules can completely defend against this serious attack. This article does not explore the technical or business impact of XSS. Suffice it to say that it can lead to an attacker gaining the ability to do anything a victim can do through their browser.

Both reflected and stored XSS can be addressed by performing the appropriate validation and escaping on the server-side. DOM Based XSS can be addressed with a special subset of rules described in the DOM based XSS Prevention Cheat Sheet.

For a cheatsheet on the attack vectors related to XSS, please refer to the XSS Filter Evasion Cheat Sheet. More background on browser security and the various browsers can be found in the Browser Security Handbook.

Before reading this cheatsheet, it is important to have a fundamental understanding of Injection Theory.

A Positive XSS Prevention Model

This article treats an HTML page like a template, with slots where a developer is allowed to put untrusted data. These slots cover the vast majority of the common places where a developer might want to put untrusted data. Putting untrusted data in other places in the HTML is not allowed. This is a "whitelist" model, that denies everything that is not specifically allowed.

Given the way browsers parse HTML, each of the different types of slots has slightly different security rules. When you put untrusted data into these slots, you need to take certain steps to make sure that the data does not break out of that slot into a context that allows code execution. In a way, this approach treats an HTML document like a parameterized database query - the data is kept in specific places and is isolated from code contexts with escaping.

This document sets out the most common types of slots and the rules for putting untrusted data into them safely. Based on the various specifications, known XSS vectors, and a great deal of manual testing with all the popular browsers, we have determined that the rule proposed here are safe.

The slots are defined and a few examples of each are provided. Developers SHOULD NOT put data into any other slots without a very careful analysis to ensure that what they are doing is safe. Browser parsing is extremely tricky and many innocuous looking characters can be significant in the right context.

Why Can't I Just HTML Entity Encode Untrusted Data?

HTML entity encoding is okay for untrusted data that you put in the body of the HTML document, such as inside a <div> tag. It even sort of works for untrusted data that goes into attributes, particularly if you're religious about using quotes around your attributes. But HTML entity encoding doesn't work if you're putting untrusted data inside a <script> tag anywhere, or an event handler attribute like onmouseover, or inside CSS, or in a URL. So even if you use an HTML entity encoding method everywhere, you are still most likely vulnerable to XSS. You MUST use the escape syntax for the part of the HTML document you're putting untrusted data into. That's what the rules below are all about.

You Need a Security Encoding Library

Writing these encoders is not tremendously difficult, but there are quite a few hidden pitfalls. For example, you might be tempted to use some of the escaping shortcuts like \" in JavaScript. However, these values are dangerous and may be misinterpreted by the nested parsers in the browser. You might also forget to escape the escape character, which attackers can use to neutralize your attempts to be safe. OWASP recommends using a security-focused encoding library to make sure these rules are properly implemented.

Microsoft provides an encoding library named the Microsoft Anti-Cross Site Scripting Library for the .NET platform. The OWASP ESAPI project has created an escaping library in a variety of languages including Java, PHP, Classic ASP, Cold Fusion, Python, and Haskell. The OWASP project also provides the OWASP Java Encoder Project and the OWASP Encoding Project.

XSS Prevention Rules

The following rules are intended to prevent all XSS in your application. While these rules do not allow absolute freedom in putting untrusted data into an HTML document, they should cover the vast majority of common use cases. You do not have to allow all the rules in your organization. Many organizations may find that allowing only Rule #1 and Rule #2 are sufficient for their needs. Please add a note to the discussion page if there is an additional context that is often required and can be secured with escaping.

Do NOT simply escape the list of example characters provided in the various rules. It is NOT sufficient to escape only that list. Blacklist approaches are quite fragile. The whitelist rules here have been carefully designed to provide protection even against future vulnerabilities introduced by browser changes.

RULE #0 - Never Insert Untrusted Data Except in Allowed Locations

The first rule is to deny all - don't put untrusted data into your HTML document unless it is within one of the slots defined in Rule #1 through Rule #5. The reason for Rule #0 is that there are so many strange contexts within HTML that the list of escaping rules gets very complicated. We can't think of any good reason to put untrusted data in these contexts. This includes "nested contexts" like a URL inside a javascript -- the encoding rules for those locations are tricky and dangerous. If you insist on putting untrusted data into nested contexts, please do a lot of cross-browser testing and let us know what you find out.

<script>...NEVER PUT UNTRUSTED DATA HERE...</script>   directly in a script
 
 <!--...NEVER PUT UNTRUSTED DATA HERE...-->             inside an HTML comment
 
 <div ...NEVER PUT UNTRUSTED DATA HERE...=test />       in an attribute name
 
 <NEVER PUT UNTRUSTED DATA HERE... href="/test" />   in a tag name
 
 <style>...NEVER PUT UNTRUSTED DATA HERE...</style>   directly in CSS

Most importantly, never accept actual JavaScript code from an untrusted source and then run it. For example, a parameter named "callback" that contains a JavaScript code snippet. No amount of escaping can fix that.

RULE #1 - HTML Escape Before Inserting Untrusted Data into HTML Element Content

Rule #1 is for when you want to put untrusted data directly into the HTML body somewhere. This includes inside normal tags like div, p, b, td, etc. Most web frameworks have a method for HTML escaping for the characters detailed below. However, this is absolutely not sufficient for other HTML contexts. You need to implement the other rules detailed here as well.

<body>...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...</body>
 
 <div>...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...</div>
 
 any other normal HTML elements

Escape the following characters with HTML entity encoding to prevent switching into any execution context, such as script, style, or event handlers. Using hex entities is recommended in the spec. In addition to the 5 characters significant in XML (&, <, >, ", '), the forward slash is included as it helps to end an HTML entity.

& --> &
 < --> &lt;
 > --> &gt;
 " --> &quot;
 ' --> &#x27;     &apos; not recommended because its not in the HTML spec (See: section 24.4.1) &apos; is in the XML and XHTML specs.
 / --> &#x2F;     forward slash is included as it helps end an HTML entity

See the ESAPI reference implementation of HTML entity escaping and unescaping.

String safe = ESAPI.encoder().encodeForHTML( request.getParameter( "input" ) );

RULE #2 - Attribute Escape Before Inserting Untrusted Data into HTML Common Attributes

Rule #2 is for putting untrusted data into typical attribute values like width, name, value, etc. This should not be used for complex attributes like href, src, style, or any of the event handlers like onmouseover. It is extremely important that event handler attributes should follow Rule #3 for HTML JavaScript Data Values.

<div attr=...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...>content</div>     inside UNquoted attribute
 
 <div attr='...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...'>content</div>   inside single quoted attribute
 
 <div attr="...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...">content</div>   inside double quoted attribute

Except for alphanumeric characters, escape all characters with ASCII values less than 256 with the &#xHH; format (or a named entity if available) to prevent switching out of the attribute. The reason this rule is so broad is that developers frequently leave attributes unquoted. Properly quoted attributes can only be escaped with the corresponding quote. Unquoted attributes can be broken out of with many characters, including [space] % * + , - / ; < = > ^ and |.

See the ESAPI reference implementation of HTML entity escaping and unescaping.

String safe = ESAPI.encoder().encodeForHTMLAttribute( request.getParameter( "input" ) );

RULE #3 - JavaScript Escape Before Inserting Untrusted Data into JavaScript Data Values

Rule #3 concerns dynamically generated JavaScript code - both script blocks and event-handler attributes. The only safe place to put untrusted data into this code is inside a quoted "data value." Including untrusted data inside any other JavaScript context is quite dangerous, as it is extremely easy to switch into an execution context with characters including (but not limited to) semi-colon, equals, space, plus, and many more, so use with caution.

<script>alert('...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...')</script>     inside a quoted string
 
 <script>x='...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...'</script>          one side of a quoted expression
 
 <div onmouseover="x='...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...'"</div>  inside quoted event handler

Please note there are some JavaScript functions that can never safely use untrusted data as input - EVEN IF JAVASCRIPT ESCAPED!

For example:

<script>
 window.setInterval('...EVEN IF YOU ESCAPE UNTRUSTED DATA YOU ARE XSSED HERE...');
 </script>

Except for alphanumeric characters, escape all characters less than 256 with the \xHH format to prevent switching out of the data value into the script context or into another attribute. DO NOT use any escaping shortcuts like \" because the quote character may be matched by the HTML attribute parser which runs first. These escaping shortcuts are also susceptible to "escape-the-escape" attacks where the attacker sends \" and the vulnerable code turns that into \\" which enables the quote.

If an event handler is properly quoted, breaking out requires the corresponding quote. However, we have intentionally made this rule quite broad because event handler attributes are often left unquoted. Unquoted attributes can be broken out of with many characters including [space] % * + , - / ; < = > ^ and |. Also, a </script> closing tag will close a script block even though it is inside a quoted string because the HTML parser runs before the JavaScript parser.

See the ESAPI reference implementation of JavaScript escaping and unescaping.

String safe = ESAPI.encoder().encodeForJavaScript( request.getParameter( "input" ) );

RULE #3.1 - HTML escape JSON values in an HTML context and read the data with JSON.parse

In a Web 2.0 world, the need for having data dynamically generated by an application in a javascript context is common. One strategy is to make an AJAX call to get the values, but this isn't always performant. Often, an initial block of JSON is loaded into the page to act as a single place to store multiple values. This data is tricky, though not impossible, to escape correctly without breaking the format and content of the values.

Ensure returned Content-Type header is application/json and not text/html. This shall instruct the browser not misunderstand the contect and execute injected script

Bad HTTP response:

HTTP/1.1 200
   Date: Wed, 06 Feb 2013 10:28:54 GMT
   Server: Microsoft-IIS/7.5....
   Content-Type: text/html; charset=utf-8 <-- bad
   ....
   Content-Length: 373
   Keep-Alive: timeout=5, max=100
   Connection: Keep-Alive
   {"Message":"No HTTP resource was found that matches the request URI 'dev.net.ie/api/pay/.html?HouseNumber=9&AddressLine
   =The+Gardens<script>alert(1)</script>&AddressLine2=foxlodge+woods&TownName=Meath'.","MessageDetail":"No type was found
   that matches the controller named 'pay'."}   <-- this script will pop!!

Good HTTP response

HTTP/1.1 200
   Date: Wed, 06 Feb 2013 10:28:54 GMT
   Server: Microsoft-IIS/7.5....
   Content-Type: application/json; charset=utf-8 <--good
   .....
   .....

A common anti-pattern one would see:

<script>
     var initData = <%= data.to_json %>; // Do NOT do this.
   </script>

Instead, consider placing the JSON block on the page as a normal element and then parsing the innerHTML to get the contents. The javascript that reads the span can live in an external file, thus making the implementation of CSP enforcement easier.

<script id="init_data" type="application/json">
    <%= data.to_json %>  <-- data is HTML escaped, or at least json entity escaped -->
 </script>

<script>
   var jsonText = document.getElementById('init_data').innerHTML;  // unescapes the content of the span
   var initData = JSON.parse(jsonText); 
 </script>

The data is added to the page and is HTML entity escaped so it won't pop in the HTML context. The data is then read by innerHTML, which unescapes the value. The unescaped text from the page is then parsed with JSON.parse.

RULE #4 - CSS Escape And Strictly Validate Before Inserting Untrusted Data into HTML Style Property Values

Rule #4 is for when you want to put untrusted data into a stylesheet or a style tag. CSS is surprisingly powerful, and can be used for numerous attacks. Therefore, it's important that you only use untrusted data in a property value and not into other places in style data. You should stay away from putting untrusted data into complex properties like url, behavior, and custom (-moz-binding). You should also not put untrusted data into IE’s expression property value which allows JavaScript.

<style>selector { property : ...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...; } </style>     property value

 <style>selector { property : "...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE..."; } </style>   property value

 <span style="property : ...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...">text</style>       property value

Please note there are some CSS contexts that can never safely use untrusted data as input - EVEN IF PROPERLY CSS ESCAPED! You will have to ensure that URLs only start with "http" not "javascript" and that properties never start with "expression".

For example:

{ background-url : "javascript:alert(1)"; }  // and all other URLs
 { text-size: "expression(alert('XSS'))"; }   // only in IE

Except for alphanumeric characters, escape all characters with ASCII values less than 256 with the \HH escaping format. DO NOT use any escaping shortcuts like \" because the quote character may be matched by the HTML attribute parser which runs first. These escaping shortcuts are also susceptible to "escape-the-escape" attacks where the attacker sends \" and the vulnerable code turns that into \\" which enables the quote.

If attribute is quoted, breaking out requires the corresponding quote. All attributes should be quoted but your encoding should be strong enough to prevent XSS when untrusted data is placed in unquoted contexts. Unquoted attributes can be broken out of with many characters including [space] % * + , - / ; < = > ^ and |. Also, the </style> tag will close the style block even though it is inside a quoted string because the HTML parser runs before the JavaScript parser. Please note that we recommend aggressive CSS encoding and validation to prevent XSS attacks for both quoted and unquoted attributes.

See the ESAPI reference implementation of CSS escaping and unescaping.

String safe = ESAPI.encoder().encodeForCSS( request.getParameter( "input" ) );

RULE #5 - URL Escape Before Inserting Untrusted Data into HTML URL Parameter Values

Rule #5 is for when you want to put untrusted data into HTTP GET parameter value.

<a href="http://www.somesite.com?test=...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...">link</a >

Except for alphanumeric characters, escape all characters with ASCII values less than 256 with the %HH escaping format. Including untrusted data in data: URLs should not be allowed as there is no good way to disable attacks with escaping to prevent switching out of the URL. All attributes should be quoted. Unquoted attributes can be broken out of with many characters including [space] % * + , - / ; < = > ^ and |. Note that entity encoding is useless in this context.

See the ESAPI reference implementation of URL escaping and unescaping.

String safe = ESAPI.encoder().encodeForURL( request.getParameter( "input" ) );

WARNING: Do not encode complete or relative URL's with URL encoding! If untrusted input is meant to be placed into href, src or other URL-based attributes, it should be validated to make sure it does not point to an unexpected protocol, especially Javascript links. URL's should then be encoded based on the context of display like any other piece of data. For example, user driven URL's in HREF links should be attribute encoded. For example:

String userURL = request.getParameter( "userURL" )
 boolean isValidURL = ESAPI.validator().isValidInput("URLContext", userURL, "URL", 255, false); 
 if (isValidURL) {  
     <a href="<%=encoder.encodeForHTMLAttribute(userURL)%>">link</a>
 }

RULE #6 - Use an HTML Policy engine to validate or clean user-driven HTML in an outbound way

OWASP AntiSamy

import org.owasp.validator.html.*;
  Policy policy = Policy.getInstance(POLICY_FILE_LOCATION);
  AntiSamy as = new AntiSamy();
  CleanResults cr = as.scan(dirtyInput, policy);
  MyUserDAO.storeUserProfile(cr.getCleanHTML()); // some custom function

OWASP Java HTML Sanitizer

import org.owasp.html.Sanitizers;
  import org.owasp.html.PolicyFactory;
  PolicyFactory sanitizer = Sanitizers.FORMATTING.and(Sanitizers.BLOCKS);
  String cleanResults = sanitizer.sanitize("<p>Hello, <b>World!</b>");

For more information on OWASP Java HTML Sanitizer policy construction, see http://owasp-java-html-sanitizer.googlecode.com/svn/trunk/distrib/javadoc/org/owasp/html/Sanitizers.html

RULE #7 - Prevent DOM-based XSS

For details on what DOM-based XSS is, and defenses against this type of XSS flaw, please see the OWASP article on DOM based XSS Prevention Cheat Sheet.

Bonus Rule: Use HTTPOnly cookie flag

Preventing all XSS flaws in an application is hard, as you can see. To help mitigate the impact of an XSS flaw on your site, OWASP also recommends you set the HTTPOnly flag on your session cookie and any custom cookies you have that are not accessed by any Javascript you wrote. This cookie flag is typically on by default in .NET apps, but in other languages you have to set it manually. For more details on the HTTPOnly cookie flag, including what it does, and how to use it, see the OWASP article on HTTPOnly.

XSS Prevention Rules Summary

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

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 Rules Summary

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.