BGP TCP vulnerability - Update (posted 2004-04-22)
It is becoming clear that there are indeed systems that are vulnerable to having TCP sessions reset within only four tries, assuming IP addresses and port numbers are known. Unfortunately, there is little information about which systems have this vulnerability. However, judging from the secrecy at Cisco and Juniper, it is far from inconceivable that they are vulnerable. (If my suspicions are correct, the hole was fixed in FreeBSD in 1998 (!!!), though.)
The details will probably be public on thursday, and we can expect exploits very soon after that. Since the required number of packets to take advantage of the vulnerability is very low, having MD5 in place on BGP sessions is almost certainly a good idea, as it is unlikely a router will receive so many packets that the CPU is overloaded. Also, filtering BGP RSTs as outlined below/above where possible will make sure your routers won't terminate TCP sessions. However, your sessions may still be vulnerable depending on the status of your BGP neighbor's router.
So set up MD5 passwords on important BGP sessions (such as the ones to transit networks) as soon as possible.
The Cisco advisory on just the problem with RSTs and the window. If I interpret this correctly, fixed IOS versions are already available, even to Cisco users without support contracts. (Note that non-IOS products are also affected, and it's a good idea to upgrade anyway as per Cisco's recently uncovered SNMP vulnerability.)