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.
https://www.ubuntu.com/usn/usn-3255-1/
https://www.exploit-db.com/exploits/41923/
https://lists.freedesktop.org/archives/lightdm/2017-April/001059.html
https://launchpad.net/bugs/1677924
http://www.securityfocus.com/bid/97486
http://bazaar.launchpad.net/~lightdm-team/lightdm/trunk/revision/2478