It is possible to deduce if a file exists or not by exploiting
the way that Xorg creates its lock files. This is caused by the
fact that the X server is behaving differently if the lock file
already exists as a symbolic link pointing to an existing or
non-existing file.
It is possible for a non-root user to set the permissions for
all users on any file or directory to 444, giving unwanted read
access or causing denies of service (by removing execute
permission). This is caused by a race between creating the lock
file and setting its access modes.