This lab has a "Check stock" feature that parses XML input and returns any unexpected values in the response.
The lab server is running a (simulated) EC2 metadata endpoint at the default URL, which is http://169.254.169.254/. This endpoint can be used to retrieve data about the instance, some of which might be sensitive.
To solve the lab, exploit the XXE vulnerability to perform an SSRF attack that obtains the server's IAM secret access key from the EC2 metadata endpoint.
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 test [ <!ENTITY xxe SYSTEM "http://169.254.169.254/"> ]>
Then replace the productId number with a reference to the external entity: &xxe;
The response should contain "Invalid product ID:" followed by the response from the metadata endpoint, which will initially be a folder name. Iteratively update the URL in the DTD to explore the API until you reach /latest/meta-data/iam/security-credentials/admin. This should return JSON containing the SecretAccessKey.