The LockServer function in os/utils.c in X.Org xserver before 1.11.2 allows local users to determine the existence of arbitrary files via a symlink attack on a temporary lock file, which is handled differently if the file exists.
Exploitability
AV:LAC:HAu:NImpact
C:PI:NA:N1.2/AV:L/AC:H/Au:N/C:P/I:N/A:N