In systemd before v242-rc4, 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".
https://www.exploit-db.com/exploits/46743/
https://lists.debian.org/debian-lts-announce/2019/04/msg00022.html
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2019-3842
http://packetstormsecurity.com/files/152610/systemd-Seat-Verification-Active-Session-Spoofing.html
http://lists.opensuse.org/opensuse-security-announce/2019-05/msg00062.html