OpenBSD before 20070116 allows remote attackers to cause a denial of service (infinite loop and CPU consumption) via certain IPv6 ICMP (aka ICMP6) echo request packets.
http://www.securityfocus.com/bid/22087
http://www.openbsd.org/errata39.html#icmp6
http://www.openbsd.org/errata.html#icmp6