SSRF with whitelist-based input filter

Description

This lab has a 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 http://127.0.0.1/ and observe that the application is parsing the URL, extracting the hostname, and validating it against a whitelist.

3. Change the URL to http://username@stock.weliketoshop.net/ and observe that this is accepted, indicating that the URL parser supports embedded credentials.

4. Append a # to the username and observe that the URL is now rejected.

5. Double-URL encode the # to %2523 and observe the extremely suspicious “Internal Server Error” response, indicating that the server may have attempted to connect to username.

6. To access the admin interface and delete the target user, change the URL to:

Exploitability

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 an anti-SSRF defense the attacker will need to bypass.