29.1 Lab: Client-side prototype pollution via browser APIs

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 | Karthikeyan Nagaraj

Karthikeyan Nagaraj
2 min readJun 11, 2024

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.

To solve the lab:

  1. Find a source that you can use to add arbitrary properties to the global Object.prototype.
  2. Identify a gadget property that allows you to execute arbitrary JavaScript.
  3. Combine these to call alert().

You can solve this lab manually in your browser, or use DOM Invader to help you.

This lab is based on real-world vulnerabilities discovered by PortSwigger Research. For more details, check out Widespread prototype pollution gadgets by Gareth Heyes.

Solution

  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).

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Karthikeyan Nagaraj

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