GNOME Display Manager (gdm) 3.4.1 and earlier, when disable-user-list is set to true, allows local users to cause a denial of service (unable to login) by pressing the cancel button after entering a user name.
https://bugzilla.redhat.com/show_bug.cgi?id=1050745
https://bugzilla.gnome.org/show_bug.cgi?id=704284
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=683338