The mountpoint_last function in fs/namei.c in the Linux kernel before 3.15.8 does not properly maintain a certain reference count during attempts to use the umount system call in conjunction with a symlink, which allows local users to cause a denial of service (memory consumption or use-after-free) or possibly have unspecified other impact via the umount program.
The init.d script for the X.Org X11 xfs font server on various Linux distributions might allow local users to change the permissions of arbitrary files via a symlink attack on the /tmp/.font-unix temporary file.