The /api/xssi endpoint is vulnerable to XSSI. The endpoint returns an application/javascript response which sets var secret = "s3cr3t".

Requests in violation of the Resource Isolation Policy will be rejected with a 403 error.

You win if you exploit the XSSI bug and read the contents of the secret variable from a cross-origin attack page on a browser with Fetch Metadata enabled.

data:text/html,<script src=""> </script>