The IKEv1 state machine in Cisco IOS 15.4 through 15.6 and IOS XE 3.15 through 3.17 allows remote attackers to cause a denial of service (IPsec connection termination) via a crafted IKEv1 packet to a tunnel endpoint, aka Bug ID CSCuw08236.
http://www.securitytracker.com/id/1034505
http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20151218-ios