Information
Leaving the user list enabled is a security risk as it allows anyone with physical access to the system to enumerate known user accounts without authenticated access to the system.
Solution
Configure the operating system to disable the login screen user list for graphical user interfaces.
Create or edit the gdm profile in '/etc/dconf/profile/' to contain the following lines:
$ sudo vi /etc/dconf/profile/gdm
user-db:user
system-db:gdm
file-db:/usr/share/gdm/greeter-dconf-defaults
Create or edit the gdm database for machine-wide settings in '/etc/dconf/db/gdm.d/' with the following lines:
$ sudo vi /etc/dconf/db/gdm.d/00-login-screen
[org/gnome/login-screen]
disable-user-list=true
Update the system databases by updating the dconf utility:
$ sudo dconf update
If the login screen user list persists after updating the system databases, you can restart the GNOME Desktop without rebooting the system:
$ sudo systemctl restart gdm