Gotify server XSS vulnerability in the application image file upload

Summary

Gotify server is a simple server for sending and receiving messages in real-time per WebSocket. Versions prior to 2.2.2 contain an XSS vulnerability that allows authenticated users to upload .html files. An attacker could execute client side scripts **if** another user opened a link. The attacker could potentially take over the account of the user that clicked the link. The Gotify UI won't natively expose such a malicious link, so an attacker has to get the user to open the malicious link in a context outside of Gotify. The vulnerability has been fixed in version 2.2.2. As a workaround, you can block access to non image files via a reverse proxy in the `./image` directory.