include/net/netfilter/nf_conntrack_extend.h in the netfilter subsystem in the Linux kernel before 3.14.5 uses an insufficiently large data type for certain extension data, which allows local users to cause a denial of service (NULL pointer dereference and OOPS) via outbound network traffic that triggers extension loading, as demonstrated by configuring a PPTP tunnel in a NAT environment.
http://marc.info/?l=netfilter-devel&m=140112364215200&w=2
http://rhn.redhat.com/errata/RHSA-2015-1534.html
http://rhn.redhat.com/errata/RHSA-2015-1564.html
https://bugzilla.redhat.com/show_bug.cgi?id=1208684
https://github.com/torvalds/linux/commit/223b02d923ecd7c84cf9780bb3686f455d279279
http://www.debian.org/security/2015/dsa-3237
http://www.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.14.5
http://www.openwall.com/lists/oss-security/2015/04/08/1
http://www.oracle.com/technetwork/topics/security/linuxbulletinjan2016-2867209.html