LightDM before 1.0.6 allows local users to change ownership of arbitrary files via a symlink attack on ~/.Xauthority.
http://www.ubuntu.com/usn/USN-1262-1
http://www.openwall.com/lists/oss-security/2011/11/02/9
http://www.openwall.com/lists/oss-security/2011/11/02/6
http://www.openwall.com/lists/oss-security/2011/11/02/10
http://lists.freedesktop.org/archives/lightdm/2011-November/000178.html