IPSEC implementations including (1) FreeS/WAN and (2) KAME do not properly calculate the length of authentication data, which allows remote attackers to cause a denial of service (kernel panic) via spoofed, short Encapsulating Security Payload (ESP) packets, which result in integer signedness errors.
http://www.securityfocus.com/bid/6011
http://www.kb.cert.org/vuls/id/459371
http://www.iss.net/security_center/static/10411.php