include/linux/init_task.h in the Linux kernel before 2.6.35 does not prevent signals with a process group ID of zero from reaching the swapper process, which allows local users to cause a denial of service (system crash) by leveraging access to this process group.
https://github.com/torvalds/linux/commit/fa2755e20ab0c7215d99c2dc7c262e98a09b01df
https://github.com/torvalds/linux/commit/f20011457f41c11edb5ea5038ad0c8ea9f392023
https://github.com/torvalds/linux/commit/f106eee10038c2ee5b6056aaf3f6d5229be6dcdd
https://bugzilla.redhat.com/show_bug.cgi?id=1358840
http://www.securityfocus.com/bid/97103