rpcrt4.dll (aka the RPC runtime library) in Microsoft Windows XP SP2, XP Professional x64 Edition, Server 2003 SP1 and SP2, Server 2003 x64 Edition and x64 Edition SP2, and Vista and Vista x64 Edition allows remote attackers to cause a denial of service (RPCSS service stop and system restart) via an RPC request that uses NTLMSSP PACKET authentication with a zero-valued verification trailer signature, which triggers an invalid dereference.  NOTE: this also affects Windows 2000 SP4, although the impact is an information leak.