This update resolves the 'Malformed PPTP Packet Stream' security vulnerability in Windows NT 4.0. Download now to prevent a malicious user from causing your server to stop responding or fail. The Point-to-Point Tunneling Protocol (PPTP) is a network service through which a local area network (LAN) connects to the Internet. The PPTP service in Windows NT 4.0 incorrectly processes a particular type of data packet. If a large number of specifically malformed data packets are sent to an affected server, it can exhaust the server's memory and cause the server to stop responding or fail. If this occurs, any PPTP sessions in progress are lost.