The TCP/IP stack in multiple operating systems allows remote attackers to cause a denial of service (CPU consumption) via a TCP packet with the correct sequence number but the wrong Acknowledgement number, which generates a large number of "keep alive" packets. NOTE: some followups indicate that this issue could not be replicated.
https://exchange.xforce.ibmcloud.com/vulnerabilities/40502
http://www.securityfocus.com/bid/13215
http://seclists.org/lists/fulldisclosure/2005/Apr/0385.html