lightdm before 0.9.6 writes in .dmrc and Xauthority files using root permissions while the files are in user controlled folders. A local user can overwrite root-owned files via a symlink, which can allow possible privilege escalation.

Array index error in LightDM (aka Light Display Manager) 1.14.3, 1.16.x before 1.16.6 when the XDMCP server is enabled allows remote attackers to cause a denial of service (process crash) via an XDMCP request packet with no address.

LightDM through 1.22.0, when systemd is used in Ubuntu 16.10 and 17.x, allows physically proximate attackers to bypass intended AppArmor restrictions and visit the home directories of arbitrary users by establishing a guest session.

In LightDM through 1.22.0, a directory traversal issue in debian/guest-account.sh allows local attackers to own arbitrary directory path locations and escalate privileges to root when the guest user logs out.

The start_authentication function in lightdm-gtk-greeter.c in LightDM GTK+ Greeter before 1.7.1 does not properly handle the return value from the lightdm_greeter_get_authentication_user function, which allows local users to cause a denial of service (NULL pointer dereference) via an empty username.