VMware vCenter Server 4.0 before Update 4b, 5.0 before Update 2, and 5.1 before 5.1.0b; VMware ESXi 3.5 through 5.1; and VMware ESX 3.5 through 4.1 do not properly implement the Network File Copy (NFC) protocol, which allows man-in-the-middle attackers to execute arbitrary code or cause a denial of service (memory corruption) by modifying the client-server data stream.