If you are running Overseerr behind a reverse proxy, make sure that the Enable Proxy Support setting is enabled.

To use Fail2ban with Overseerr, create a new file named overseerr.local in your Fail2ban filter.d directory with the following filter definition:

failregex = .*\[info\]\[Auth\]\: Failed sign-in attempt.*"ip":"<HOST>"

You can then add a jail using this filter in jail.local. Please see the Fail2ban documetation for details on how to configure the jail.