kernel/user_namespace.c in the Linux kernel before 3.8.9 does not have appropriate capability requirements for the uid_map and gid_map files, which allows local users to gain privileges by opening a file within an unprivileged process and then modifying the file within a privileged process.
https://github.com/torvalds/linux/commit/e3211c120a85b792978bcb4be7b2886df18d27f0
https://github.com/torvalds/linux/commit/6708075f104c3c9b04b23336bb0366ca30c3931b
http://www.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.8.9