This is a Server-Side Request Forgery (SSRF) vulnerability in the PaperCut NG/MF server-side module that allows an attacker to induce the server-side application to make HTTP requests to an arbitrary domain of the attackers choosing.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.