Visit a product, click "Check stock", intercept the request in Burp Suite, and send it to Burp Repeater.
Try tampering with the stockApi parameter and observe that it isn't possible to make the server issue the request directly to a different host.
Click "next product" and observe that the path parameter is placed into the Location header of a redirection response, resulting in an open redirection.
Create a URL that exploits the open redirection vulnerability, and redirects to the admin interface, and feed this into the stockApi parameter on the stock checker: /product/nextProduct?path=http://192.168.0.12:8080/admin
The stock checker should follow the redirection and show you the admin page. You can then amend the path to delete the target user: /product/nextProduct?path=http://192.168.0.12:8080/admin/delete?username=carlos
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