Testing for JavaScript Execution

Brief Summary
A JavaScript Injection vulnerability is a subtype of Cross Site Scripting (XSS) which involves the ability to inject arbitrary JavaScript code that is executed by the application inside the victim's browser. This vulnerability can have many consequences, like disclosure of a user's session cookies that could be used to impersonate the victim, or, more generally, it can allow the attacker to modify the page content seen by the victims or the application behavior.

Description of the Issue
Such vulnerability occurs when the application lacks of a proper user supplied input and output validation. JavaScript is used to dynamically populate web pages, this injection occur during this content processing phase and consequently affect the victim. Exploitation Notes:

When trying to exploit this kind of issues, consider that some character is treated differently by different browsers. For reference see DOM XSS Wiki

The following script does not perform any validation of the variable rr that contains the user supplied input via the query string and additionally does not apply any form of encoding:

var rr = location.search.substring(1); if(rr) window.location=decodeURIComponent(rr); This implies that an attacker could inject JavaScript code simply by submitting the following query string: www.victim.com/?javascript:alert(1)

Black Box testing and example
Blackbox testing for JavaScript Execution is not usually performed since access to the source code is always available as it needs to be sent to the client to be executed.

Gray Box testing and example
Testing for JavaScript Execution vulnerabilities: For example, looking at the following URL: http://www.domxss.com/domxss/01_Basics/04_eval.html

The page contains the following scripts:

function loadObj{ var cc=eval('('+aMess+')'); document.getElementById('mess').textContent=cc.message; }

if(window.location.hash.indexOf('message')==-1) var aMess="({\"message\":\"Hello User!\"})"; else var aMess=location.hash.substr(window.location.hash.indexOf('message=')+8);

The above code contains a source 'location.hash' that is controlled by the attacker that can inject directly in the 'message' value a JavaScript Code to take the control of the user browser.