CVE-2019-3842: Spoofing of XDG_SEAT allows for actions to be checked against "allow_active" instead of "allow_any"

Description Sam Fowler 2019-01-23 01:23:07 UTC

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.

Comment 1 Riccardo Schirone 2019-03-01 15:44:01 UTC

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.

Comment 2 Riccardo Schirone 2019-03-01 15:49:10 UTC

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.

Comment 10 Riccardo Schirone 2019-04-09 13:56:21 UTC

Created systemd tracking bugs for this issue:

Affects: fedora-all [bug 1698045]

Comment 11 Riccardo Schirone 2019-04-09 15:30:06 UTC

Acknowledgments:

Name: Jann Horn (Google Project Zero)

Comment 13 Riccardo Schirone 2020-03-06 13:57:55 UTC

Statement:

For the attack to be successful, a new session must be created by pam_systemd. This is done only if the calling process is not already part of a session. Red Hat Enterprise Linux, in its default PAM configurations, does not let a session sneak in without systemd knowing about it, since pam_systemd is always called in every PAM config file. Unless a wrong PAM config file is in place, this vulnerability cannot be triggered on Red Hat Enterprise Linux 7 and 8.