The implementation of SYN cookies (syncookies) in FreeBSD 4.5 through 5.0-RELEASE-p3 uses only 32-bit internal keys when generating syncookies, which makes it easier for remote attackers to conduct brute force ISN guessing attacks and spoof legitimate traffic.
https://exchange.xforce.ibmcloud.com/vulnerabilities/11397
http://www.securityfocus.com/bid/6920