The kill_something_info function in kernel/signal.c in the Linux kernel before 2.6.28 does not consider PID namespaces when processing signals directed to PID -1, which allows local users to bypass the intended namespace isolation, and send arbitrary signals to all processes in all namespaces, via a kill command.
git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git%3Ba=commit%3Bh=d25141a818383b3c3b09f065698c544a7a0ec6e7
lkml.org/lkml/2008/7/23/148
secunia.com/advisories/34981
secunia.com/advisories/35120
secunia.com/advisories/35121
secunia.com/advisories/35343
secunia.com/advisories/35656
wiki.rpath.com/Advisories:rPSA-2009-0084
www.debian.org/security/2009/dsa-1787
www.debian.org/security/2009/dsa-1800
www.kernel.org/pub/linux/kernel/v2.6/ChangeLog-2.6.28
www.openwall.com/lists/oss-security/2009/04/16/2
www.openwall.com/lists/oss-security/2009/04/17/4
www.openwall.com/lists/oss-security/2009/04/21/1
www.redhat.com/support/errata/RHSA-2009-1081.html
www.securityfocus.com/archive/1/503610/100/0/threaded
www.ubuntu.com/usn/usn-793-1
bugzilla.redhat.com/show_bug.cgi?id=496031
exchange.xforce.ibmcloud.com/vulnerabilities/50386