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.
Improper Link Resolution Before File Access The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.