It was discovered that pam_systemd does not properly sanitize the environment before using the XDG_SEAT variable. It is possible for an attacker, in some particular configurations, to set a XDG_SEAT environment variable which allows for commands to be checked against polkit policies using the "allow_active" element rather than "allow_any".
systemd has a vulnerability in the PAM module, pam_systemd, that allows for spoofing of the XDG_SEAT environment variable which allows for commands to be checked against polkit policies using the "allow_active" element rather than "allow_any". Users with local access to machines with an active tty sessions can exploit this to elevate their privileges.
pam_systemd uses getenv() to retrieve XDG_SEAT value, however when pam_systemd is used by a SUID binary this allows a unprivileged user, in some circumstances, to provide a fake XDG_SEAT value, with the consequences mentioned in comment 0.
For the attack to be successful, a new session must be created and that is created by pam_systemd only if the calling process is not already part of a session. On Fedora/RHEL, in their default PAM configurations, it does not seem to be possible to let a session sneak in without systemd knowing about it, since pam_systemd is always called in every PAM config file.
Created systemd tracking bugs for this issue:
Affects: fedora-all [bug 1698045]
Name: Jann Horn (Google Project Zero)