A patch that eliminates a security vulnerability in Windows 95/98/NT/2000. The vulnerability could be used to cause an affected machine to temporarily stop performing useful work. The affected systems contain a flaw in the code that performs IP fragment reassembly. If a continuous stream of fragmented IP datagrams with a particular malformation were sent to an affected machine, it could be made to devote most or all of its CPU availability to processing them. The data rate needed to completely deny service varies depending on the machine and network conditions, but in most cases even relatively moderate rates would suffice. The vulnerability would not allow a malicious user to compromise data on the machine or usurp administrative control over it. Although it has been reported that the attack in some cases will cause an affected machine to crash, affected machines in all Microsoft testing returned to normal service shortly after the fragments stopped arriving. Machines protected by a proxy server or a firewall that drops fragmented packets would not be affected by this vulnerability. The machines most likely to be affected by this vulnerability would be machines located on the edge of a network such as web servers or proxy servers.