The DHCPv4 server in Cisco IOS XR 5.2.2 on ASR 9000 devices allows remote attackers to cause a denial of service (service outage) via a flood of crafted DHCP packets, aka Bug ID CSCup67822.
http://www.securitytracker.com/id/1031970
http://tools.cisco.com/security/center/viewAlert.x?alertId=38006