Client-side prototype pollution via browser APIs

Description

This lab is vulnerable to DOM XSS via client-side prototype pollution. The website’s developers have noticed a potential gadget and attempted to patch it. However, you can bypass the measures they’ve taken. Also see Widespread prototype pollution gadgets.

Reproduction and proof of concept

1. Load the lab in Burp’s built-in browser.

2. Enable DOM Invader and enable the prototype pollution option.

3. Open the browser DevTools panel, go to the DOM Invader tab, then reload the page.

4. Observe that DOM Invader has identified two prototype pollution vectors in the search property i.e. the query string.

5. Click Scan for gadgets. A new tab opens in which DOM Invader begins scanning for gadgets using the selected source.

6. When the scan is complete, open the DevTools panel in the same tab as the scan, then go to the DOM Invader tab.

7. Observe that DOM Invader has successfully accessed the script.src sink via the value gadget.

8. Click Exploit. DOM Invader automatically generates a proof-of-concept exploit and calls alert(1).

Exploitability

An attacker will need to find a source that you can use to add arbitrary properties to the global Object.prototype; identify a gadget property that allows you to execute arbitrary JavaScript; and combine these to call alert().