isakmpd in OpenBSD 3.4 and earlier allows remote attackers to cause a denial of service (infinite loop) via an ISAKMP packet with a zero-length payload, as demonstrated by the Striker ISAKMP Protocol Test Suite.
https://exchange.xforce.ibmcloud.com/vulnerabilities/15518
http://www.securitytracker.com/alerts/2004/Mar/1009468.html
http://www.securityfocus.com/bid/10028
http://www.openbsd.org/errata.html
http://www.kb.cert.org/vuls/id/349113