From Microsoft: This update resolves the 'Malformed RPC Packet' security vulnerability in computers running Windows NT 4.0 and is discussed in Microsoft Security Bulletin MS01-048. Download now to prevent a malicious user from launching a denial of service attack using the Remote Procedure Call (RPC) client. This vulnerability exists because there is an error in the way the endpoint mapper (used by the RPC service to determine which remote port to use) processes queries. If a malicious user sends a malformed RPC query to an affected Windows NT 4.0 computer, it can cause the server to stop responding to requests.
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