Selective Acknowledgement (SACK) in FreeBSD 5.3 and 5.4 does not properly handle an incoming selective acknowledgement when there is insufficient memory, which might allow remote attackers to cause a denial of service (infinite loop).
https://exchange.xforce.ibmcloud.com/vulnerabilities/24453
http://www.vupen.com/english/advisories/2006/0409
http://www.securityfocus.com/bid/16466
http://securitytracker.com/id?1015566