VMware ESXi 4.0 through 5.1, and ESX 4.0 and 4.1, does not properly implement the Network File Copy (NFC) protocol, which allows man-in-the-middle attackers to cause a denial of service (unhandled exception and application crash) by modifying the client-server data stream.
VMware Security Advisory 2013-0011 - VMware has updated VMware ESXi and ESX to address a vulnerability in an unhandled exception in the NFC protocol handler.