The main branch is the place where security matters. Production forks of this repo (other servers) should update their servers ASAP once security issues has been resolved.
Create a new issue. Please include information on where the vulnerability is, how it's exploited and anything else that you can provide. Issues tagged with security have the hightest priority.
If the issue is a major problem that needs addressed ASAP, you can also contact me on Discord @nfoert. If I'm not replying and you have time to help out, if possible please try to contribute a fix or a patch, and let me know your progress.