Memory leak in the NAT feature in Cisco IOS 12.4, 15.0, and 15.1 allows remote attackers to cause a denial of service (memory consumption, and device hang or reload) via SIP packets that require translation, related to a "memory starvation vulnerability," aka Bug ID CSCti35326.
https://exchange.xforce.ibmcloud.com/vulnerabilities/74432
http://www.securitytracker.com/id?1026864
http://www.securityfocus.com/bid/52758
http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20120328-nat