DOM Based Cross Site Scripting

Description:

This type of attack occurs because the client-side javascript has access to the Document Object Model (DOM) rendered as a part of the response.Client Side Javascript may pe performing some calulation,manipuation,etc on a part of the DOM and rendring it on the webpage to make it look dynamic.

Example : Consider the url: www.example.com/?q=hey+there

Suppose the website designer has written a javascript code that takes the value of "q" and then displays it.

<script> var url = document.location; url = unescape(url); var query = url.substring(url.indexOf(‘q=’) + 8, url.length); document.write(query) </script>

This script parses the URL and extracts the value of q and writes it to the HTML Source code.This is similar to Reflected XSS, but involves the use of client-side javascript that changes the contents of the Webpage.

Now if a malicious actor, adds a malicious payload, this causes the payload to be dynamically written to the webpage and thus making it vulnerable.

Preventive Measures:

DOM based XSS can be prevented by:

• Avoiding user controled input
• Avoiding unsafe properties like document.innerHTML, document.write and using document.innerText,document.textContent instead