Impact
The XSS vulnerability allows authenticated users to upload .html files. With that, an attacker could execute client side scripts if another user opened a link, such as:
https://push.example.org/image/[alphanumeric string].html
An attacker could potentially take over the account of the user that clicked the link. Keep in mind, 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.
Patches
The vulnerability has been fixed in version 2.2.2.
Workarounds
You can block access to non image files via a reverse proxy in the ./image directory.
References
https://github.com/gotify/server/pull/534
https://github.com/gotify/server/pull/535
Thanks to rickshang (aka 无在无不在) for discovering and reporting this bug.