kernel/signal.c in the Linux kernel before 2.6.39 allows local users to spoof the uid and pid of a signal sender via a sigqueueinfo system call.
https://bugzilla.redhat.com/show_bug.cgi?id=690028
http://www.openwall.com/lists/oss-security/2011/03/23/2
http://rhn.redhat.com/errata/RHSA-2011-0927.html
http://ftp.osuosl.org/pub/linux/kernel/v2.6/ChangeLog-2.6.39