VMware Workstation offers printer “virtualization”, allowing a Guest OS to access and print documents on printers available to the Host OS. On VMware Workstation 11.1, the virtual printer device is added by default to new VMs, and on recent Windows Hosts, the Microsoft XPS Document Writer is available as a default printer. Even if the VMware Tools are not installed in the Guest, the COM1 port can be used to talk to the Host printing Proxy.
vprintproxy.exe is launched on the Host by vmwarevmx.exe as whichever user started VMware. vmwarevmx.exe and vprintproxy.exe communicate through named pipes. When writing to COM1 in the Guest, the packets will eventually end up in vprintproxy.exe for processing.
I won’t go over the subtleties of the protocol, but basically the printer virtualization layer is a glorified file copy operation of EMFSPOOL files from the Guest to the Host. The EMFSPOOL 2 and contained EMF files are processed on the Host by vprintproxy.exe, and can be previewed 3 on the Host thanks to TPView.dll. By supplying specially crafted EMFSPOOL and EMF files to COM1, one can trigger a variety of bugs in the vprintproxy.exe process, and achieve code execution on the Host.