Such an exploit could give the attacker access to the host operating system and all other virtual machines (VMs) running on that host. Although there have been no incidents reported in the wild, VM escape is considered to be the most serious threat to virtual machine security.
Virtual machines are designed to run in self-contained, isolated environments in the host. Each VM should be, in effect, a separate system, isolated from the host operating system and any other VMs running on the same machine. The hypervisor is an intermediary between the host operating system and virtual machines. It controls the host processor and allocates resources as required to each guest operating system.
Here's Ed Skoudis' explanation of the risk:
"If the attacker can compromise the virtual machines, they will likely have control of all of the guests, since the guests are merely subsets of the program itself. Also, most virtual machines run with very high privileges on the host because a virtual machine needs comprehensive access to the host's hardware so it can then map the real hardware into virtualized hardware for the guests. Thus, compromising the virtual machine means not only that the guests are goners, but the host is also likely lost."
To minimize vulnerability to VM escape, Skoudis recommends that you:
- Keep virtual machine software patched.
- Install only the resource-sharing features that you really need.
- Keep software installations to a minimum because each program brings its own vulnerabilities.