SSRF with blacklist-based input filter


This lab has a vulnerable stock check feature which fetches data from an internal system.

Reproduction and proof of concept

  1. Visit a product, click Check stock, intercept the request in Burp Suite, and send it to Burp Repeater.

  2. Change the URL in the stockApi parameter to and observe that the request is blocked.

  3. Bypass the block by changing the URL to: http://127.1/

  4. Change the URL to http://127.1/admin and observe that the URL is blocked again.

  5. Obfuscate the a in admin by double-URL encoding it to %2561 to access the admin interface and delete the target user.



An attacker will need to change the stock check URL to access the admin interface at http://localhost/admin and delete the user carlos. The developer has deployed two weak anti-SSRF defenses that the attacker will need to bypass.