Microsoft has released a new security patch for Proxy Server and ISA Firewall

There is a flaw in the Winsock Proxy service in Microsoft Proxy Server 2.0, and the Microsoft Firewall service in ISA Server 2000, that would allow an attacker on the internal network to send a specially crafted packet that would cause the server to stop responding to internal and external requests. Receipt of such a packet would cause CPU utilization on the server to reach 100%, and thus make the server unresponsive.

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