1.10 Ensure GDM is removed or login is configured - banner-message-text

Information

GDM is the GNOME Display Manager which handles graphical login for GNOME based systems.

Rationale:

If a graphical login is not required, it should be removed to reduce the attack surface of the system. If a graphical login is required, last logged in user display should be disabled, and a warning banner should be configured.

displaying the last logged in user eliminates half of the Userid/Password equation that an unauthorized person would need to log on.

Warning messages inform users who are attempting to login to the system of their legal status regarding the system and must include the name of the organization that owns the system and any monitoring policies that are in place.

Note: If a different GUI login service is in use and required on the system, consult your documentation to disable displaying the last logged on user and apply an equivalent banner.

Solution

Edit or create the file /etc/gdm3/greeter.dconf-defaults and edit or add the following:

[org/gnome/login-screen]
banner-message-enable=true
banner-message-text='<banner message>'
disable-user-list=true

Example banner message: Authorized uses only. All activity may be monitored and reported.
Run the following command to re-load GDM on the next login or reboot:

# dpkg-reconfigure gdm3

See Also

https://workbench.cisecurity.org/files/3228