Blind XXE with out of band interaction
Description
This lab has a "Check stock" feature that parses XML input but does not display the result.
You can detect the blind XXE vulnerability by triggering out-of-band interactions with an external domain.
To solve the lab, use an external entity to make the XML parser issue a DNS lookup and HTTP request to Burp Collaborator.
Approach
After accessing the lab, I noticed a POST request sending XML data to the backend:
My initial attempt to exploit XXE by fetching local files failed. Since I had no knowledge of other backend systems, I decided to try out-of-band interaction by reaching my Burp Collaborator. I crafted a simple payload using my Burp Collaborator URL:
After sending this request and polling Burp Collaborator, I observed multiple DNS lookup requests. This successful interaction confirmed the exploit, and the lab was solved.