The LockServer function in os/utils.c in X.Org xserver before 1.11.2 allows local users to change the permissions of arbitrary files to 444, read those files, and possibly cause a denial of service (removed execution permission) via a symlink attack on a temporary lock file.
http://secunia.com/advisories/49579
http://secunia.com/advisories/46460
http://rhn.redhat.com/errata/RHSA-2012-0939.html
http://lists.freedesktop.org/archives/xorg/2011-October/053680.html
http://cgit.freedesktop.org/xorg/xserver/commit/?id=b67581cf825940fdf52bf2e0af4330e695d724a4