XSS Prevention Cheat Sheet - Revision history

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2012-09-16T17:12:25Z

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Jmanico: /* XSS Prevention Rules Summary */

2012-09-12T15:14:56Z

‎XSS Prevention Rules Summary

Jmanico: /* XSS Prevention Rules Summary */

2012-09-12T15:12:49Z

‎XSS Prevention Rules Summary

Jmanico: /* Bonus Rule: Use HTTPOnly cookie flag */

2012-09-12T15:08:16Z

‎Bonus Rule: Use HTTPOnly cookie flag

Jmanico: /* XSS Prevention Rules */

2012-09-12T15:07:47Z

‎XSS Prevention Rules

Jmanico at 19:49, 10 September 2012

2012-09-10T19:49:33Z

Jmanico at 19:47, 10 September 2012

2012-09-10T19:47:06Z

Jmanico at 19:41, 10 September 2012

2012-09-10T19:41:28Z

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Jmanico at 10:02, 9 July 2012

2012-07-09T10:02:17Z

Jmanico at 10:00, 9 July 2012

2012-07-09T10:00:45Z

← Older revision		Revision as of 15:14, 12 September 2012
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	'''''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		'''''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.
		+
		+	{\| class="wikitable"
		+	\|-
		+	! Encoding Type
		+	! Encoding Mechanism
		+	\|-
		+	\| HTML Entity Encoding
		+	\| Convert & to &amp;<br/>Convert < to &lt;<br/>Convert > to &gt;<br/>Convert " to &quot;<br/>Convert ' to &#x27;<br/>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 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 =		= Related Articles =

@@ Line 231: / Line 231: @@
 | &lt;div><span style="color:red;">UNTRUSTED HTML</span>&lt;/div>
 | <ul><li>[https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet#RULE_.236_-_Use_an_HTML_Policy_engine_to_validate_or_clean_user-driven_HTML_in_an_outbound_way HTML Validation (JSoup, AntiSamy, HTML Sanitizer)]</li></ul>
 |-
 | String
@@ Line 241: / Line 236: @@
 | TODO
 | <ul><li>[[DOM based XSS Prevention Cheat Sheet]]</li></ul>
 |}

← Older revision		Revision as of 15:08, 12 September 2012
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	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]].		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 =		= XSS Prevention Rules Summary =

← Older revision		Revision as of 15:07, 12 September 2012
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	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]].		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.
		+
		+	{\| class="wikitable nowraplinks"
		+	\|-
		+	! Data Type
		+	! Context
		+	! Code Sample
		+	! Defense
		+	\|-
		+	\| String
		+	\| HTML Body
		+	\| <span><span style="color:red;">UNTRUSTED DATA</span></span>
		+	\| <ul><li>[https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet#RULE_.231_-_HTML_Escape_Before_Inserting_Untrusted_Data_into_HTML_Element_Content HTML Entity Encoding]</li></ul>
		+	\|-
		+	\| String
		+	\| Safe HTML Attributes
		+	\| <input type="text" name="fname" value="<span style="color:red;">UNTRUSTED DATA</span>">
		+	\| <ul><li>[https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet#RULE_.232_-_Attribute_Escape_Before_Inserting_Untrusted_Data_into_HTML_Common_Attributes Aggressive HTML Entity Encoding]</li><li>Only place untrusted data into a whitelist of safe attributes (listed below).</li><li>Strictly validate unsafe attributes such as background, id and name.</ul>
		+	\|-
		+	\| String
		+	\| GET Parameter
		+	\| <a href="/site/search?value=<span style="color:red;">UNTRUSTED DATA</span>">clickme</a>
		+	\| <ul><li>[https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet#RULE_.235_-_URL_Escape_Before_Inserting_Untrusted_Data_into_HTML_URL_Parameter_Values URL Encoding]</li></ul>
		+	\|-
		+	\| String
		+	\| Untrusted URL in a SRC or HREF attribute
		+	\| <a href="<span style="color:red;">UNTRUSTED URL</span>">clickme</a><br/><iframe src="<span style="color:red;">UNTRUSTED URL</span>" />
		+	\| <ul><li>Cannonicalize input</li><li>URL Validation</li><li>Safe URL verification</li><li>Whitelist http and https URL's only ([[Avoid the JavaScript Protocol to Open a new Window]])</li><li>Attribute encoder</li></ul>
		+	\|-
		+	\| String
		+	\| CSS Value
		+	\| <div style="width: <span style="color:red;">UNTRUSTED DATA</span>;">Selection</div>
		+	\| <ul><li>[https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet#RULE_.234_-_CSS_Escape_And_Strictly_Validate_Before_Inserting_Untrusted_Data_into_HTML_Style_Property_Values Strict structural validation]<li>CSS Hex encoding<li>Good design of CSS Features</ul>
		+	\|-
		+	\| String
		+	\| JavaScript Variable
		+	\| <script>var currentValue='<span style="color:red;">UNTRUSTED DATA</span>';</script><br/><script>someFunction('<span style="color:red;">UNTRUSTED DATA</span>');</script>
		+	\| <ul><li>Ensure JavaScript variables are quoted</li><li>JavaScript Hex Encoding</li><li>JavaScript Unicode Encoding</li><li>Avoid backslash encoding (\" or \' or \\)</li></ul>
		+	\|-
		+	\| HTML
		+	\| HTML Body
		+	\| <div><span style="color:red;">UNTRUSTED HTML</span></div>
		+	\| <ul><li>[https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet#RULE_.236_-_Use_an_HTML_Policy_engine_to_validate_or_clean_user-driven_HTML_in_an_outbound_way HTML Validation (JSoup, AntiSamy, HTML Sanitizer)]</li></ul>
		+	\|-
		+	\| JavaScript
		+	\| HTML Body
		+	\| <div><span style="color:red;">UNTRUSTED JAVASCRIPT</span></div>
		+	\| <ul><li>Sandboxing with tools such as [http://code.google.com/p/jsreg/ JSReg]</li></ul>
		+	\|-
		+	\| String
		+	\| DOM XSS
		+	\| TODO
		+	\| <ul><li>[[DOM based XSS Prevention Cheat Sheet]]</li></ul>
		+	\|-
		+	\| String
		+	\| AJAX/JSON Parsing
		+	\| JSON.parse(<span style="color:red;">UNTRUSTED JSON DATA</span>)
		+	\| <ul><li>Use JSON.parse or json2.js library to parse JSON</li><li>Avoid parsing JSON with eval() as it will execute any script included with the JSON</li></ul>
		+	\|-
		+	\| String
		+	\| AJAX/XML Parsing
		+	\| TODO
		+	\| TODO
		+	\|-
		+	\| String
		+	\| Framework Protections
		+	\| <span><span style="color:red;">UNTRUSTED DATA</span></span>
		+	\| <ul><li>Content Security Policy</li><li>[https://www.owasp.org/index.php/HTML5_Security_Cheat_Sheet#Use_the_sandbox_attribute_of_an_iframe_for_untrusted_content HTML5 Frame Sandbox]</li></ul>
		+	\|-
		+	\| String
		+	\| Loading HTML fragments using AJAX
		+	\| TODO
		+	\| <ul><li>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</li></ul>
		+	\|}
		+
		+	'''''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

	= Related Articles =		= Related Articles =

← Older revision		Revision as of 19:49, 10 September 2012
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	For a cheatsheet on the attack vectors related to XSS, please refer to the excellent [http://ha.ckers.org/xss.html XSS Cheat Sheet] by RSnake. More background on browser security and the various browsers can be found in the [http://code.google.com/p/browsersec/ Browser Security Handbook].		For a cheatsheet on the attack vectors related to XSS, please refer to the excellent [http://ha.ckers.org/xss.html XSS Cheat Sheet] by RSnake. More background on browser security and the various browsers can be found in the [http://code.google.com/p/browsersec/ Browser Security Handbook].

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

	== A Positive XSS Prevention Model ==		== A Positive XSS Prevention Model ==

← Older revision		Revision as of 19:47, 10 September 2012
Line 7:		Line 7:
	For a cheatsheet on the attack vectors related to XSS, please refer to the excellent [http://ha.ckers.org/xss.html XSS Cheat Sheet] by RSnake. More background on browser security and the various browsers can be found in the [http://code.google.com/p/browsersec/ Browser Security Handbook].		For a cheatsheet on the attack vectors related to XSS, please refer to the excellent [http://ha.ckers.org/xss.html XSS Cheat Sheet] by RSnake. More background on browser security and the various browsers can be found in the [http://code.google.com/p/browsersec/ Browser Security Handbook].

−	~~== Untrusted Data ==~~
−
−	Untrusted data is most often data that comes from the HTTP request, in the form of URL parameters, form fields, headers, or cookies. But data that comes from databases, web services, and other sources is frequently untrusted from a security perspective. That is, untrusted data is input that can be manipulated to contain a web attack payload. The [[Searching for Code in J2EE/Java\|OWASP Code Review Guide]] has a decent list of methods that return untrusted data in various languages, but you should be careful about your own methods as well.
−
−	Untrusted data should always be treated as though it contains an attack. That means you should not send it '''anywhere''' without taking steps to make sure that any attacks are detected and neutralized. As applications get more and more interconnected, the likelihood of a buried attack being decoded or executed by a downstream interpreter increases rapidly.
−
−	Traditionally, [[Data Validation\|input validation]] has been the preferred approach for handling untrusted data. However, input validation is not a great solution for injection attacks. First, input validation is typically done when the data is received, before the destination is known. That means that we don't know which characters might be significant in the target interpreter. Second, and possibly even more importantly, applications must allow potentially harmful characters in. For example, should poor Mr. O'Malley be prevented from registering in the database simply because SQL considers ' a special character?
−
−	While input validation is important and should always be performed, it is not a complete solution for injection attacks. It's better to think of input validation as [[Defense in depth\|defense in depth]] and use '''escaping''' as described below as the primary defense.
−
−	~~== Escaping (aka Output Encoding) ==~~
−
−	"[http://www.w3.org/TR/charmod/#sec-Escaping Escaping]" is a technique used to ensure that characters are treated as data, not as characters that are relevant to the interpreter's parser. There are lots of different types of escaping, sometimes confusingly called output "encoding." Some of these techniques define a special "escape" character, and other techniques have a more sophisticated syntax that involves several characters.
−
−	Do not confuse output escaping with the notion of Unicode character [http://www.w3.org/TR/charmod/#sec-Digital encoding], which involves mapping a Unicode character to a sequence of bits. This level of encoding is automatically decoded, and does '''not''' defuse attacks. However, if there are misunderstandings about the intended charset between the server and browser, it may cause unintended characters to be communicated, possibly enabling XSS attacks. This is why it is still important to [http://www.w3.org/TR/charmod/#sec-Encodings specify] the Unicode character encoding (charset), such as UTF-8, for all communications.
−
−	Escaping is the primary means to make sure that untrusted data can't be used to convey an injection attack. There is '''no harm''' in escaping data properly - it will still render in the browser properly. Escaping simply lets the interpreter know that the data is not intended to be executed, and therefore prevents attacks from working.
−
−	~~== Injection Theory ==~~
−
−	[[Injection Flaws\|Injection]] is an attack that involves breaking out of a data context and switching into a code context through the use of special characters that are significant in the interpreter being used. A data context is like <div>data context</div>. If the attacker's data gets placed into the data context, they might break out like this <div>data < script>alert("attack")</script> context</div>.
−
−	XSS is a form of injection where the interpreter is the browser and attacks are buried in an HTML document. HTML is easily the worst mashup of code and data of all time, as there are so many possible places to put code and so many different valid encodings. HTML is particularly difficult because it is not only hierarchical, but also contains many different parsers (XML, HTML, JavaScript, VBScript, CSS, URL, etc...).
−
−	To really understand what's going on with XSS, you have to consider injection into the hierarchical structure of the [http://www.w3schools.com/HTMLDOM/default.asp HTML DOM]. Given a place to insert data into an HTML document (that is, a place where a developer has allowed untrusted data to be included in the DOM), there are two ways to inject code:
−
−	;Injecting UP:The most common way is to close the current context and start a new code context. For example, this is what you do when you close an HTML attribute with a "> and start a new <script> tag. This attack closes the original context (going up in the hierarchy) and then starts a new tag that will allow script code to execute. Remember that you may be able to skip many layers up in the hierarchy when trying to break out of your current context. For example, a </script> tag may be able to terminate a script block even if it is injected inside a quoted string inside a method call inside the script. This happens because the HTML parser runs before the JavaScript parser.
−
−	;Injecting DOWN:The less common way to perform XSS injection is to introduce a code subcontext without closing the current context. For example, if the attacker is able to change <img src="...UNTRUSTED DATA HERE..." /> into < img src="javascript:alert(document.cookie)" /> they do not have to break out of the HTML attribute context. Instead, they introduce a subcontext that allows scripting within the src attribute (in this case a javascript url). Another example is the expression() functionality in CSS properties. Even though you may not be able to escape a quoted CSS property to inject up, you may be able to introduce something like xss:expression(document.write(document.cookie)) without ever leaving the current context.
−
−	There's also the possibility of injecting directly in the current context. For example, if you take untrusted input and put it directly into a JavaScript context. While insane, accepting code from an attacker is more common than you might think in modern applications. Generally it is impossible to secure untrusted code with escaping (or anything else). If you do this, your application is just a conduit for attacker code to get running in your users' browsers.
−
−	The rules in this document have been designed to prevent both UP and DOWN varieties of XSS injection. To prevent injecting up, you must escape the characters that would allow you to close the current context and start a new one. To prevent attacks that jump up several levels in the DOM hierarchy, you must also escape all the characters that are significant in all enclosing contexts. To prevent injecting down, you must escape any characters that can be used to introduce a new sub-context within the current context.

	== A Positive XSS Prevention Model ==		== A Positive XSS Prevention Model ==

@@ Line 4: / Line 4: @@
 <ol>
-<li>[[XSS (Cross Site Scripting) Prevention Cheat Sheet XSS]]
+<li>[[XSS (Cross Site Scripting) Prevention Cheat Sheet]]
-<li>[[DOM_based_XSS_Prevention_Cheat_Sheet]]
+<li>[[DOM based XSS Prevention Cheat Sheet]]
-<li>[[Abridged_XSS_Prevention_Cheat_Sheet]]
+<li>[[Abridged XSS Prevention Cheat Sheet]]
 </ol>