Do you mean filesystem security or WebUI security? On web interface you can assign specific computers/backups to specific users. On the filesystem, at least on Linux, files have 750 permissions with user urbackup, so access is pretty limited unless you are the admin.
Other thing to do is encryption in transit which you can do if you designate your server as an Internet server, not a local one. Can’t remember if UrBackup does file encryption but I would guess not?