A flaw was found in GDM in versions prior to 3.38.2.1. A race condition in the handling of session shutdown makes it possible to bypass the lock screen for a user that has autologin enabled, accessing their session without authentication. This is similar to CVE-2017-12164, but requires more difficult conditions to exploit.

GNOME Display Manager (gdm) before 3.18.2 allows physically proximate attackers to bypass the lock screen by holding the Escape key.

GNOME Display Manager (gdm) before 2.21.1 allows local users to change permissions of arbitrary directories via a symlink attack on /tmp/.X11-unix/.

vicious-extensions/ve-misc.c in GNOME Display Manager (gdm) 2.20.x before 2.20.11, when GDM debug is enabled, logs the user password when it contains invalid UTF8 encoded characters, which might allow local users to gain privileges by reading the information from syslog logs.

It was discovered that the GNOME Display Manager (gdm) cleared the cache directory, which is owned by an unprivileged user, with the privileges of the root user. A race condition exists in gdm where a local user could take advantage of this by writing to the cache directory between ending the session and the signal to clean up the session, which could lead to the execution of arbitrary code as the root user.

Acknowledgements:

Red Hat would like to thank Sebastian Krahmer of the SUSE Security Team for reporting this issue.