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While virtual machines are at risk of all the same things as any other machine, you should be aware of a few additional issues.
- If a host is compromised, scripts can be run on the host that can interact with the guest at whatever privilege level the guest is logged in as. This can result in malicious trojans being installed on the host and guest machines.
- A virtual machine that is not virus protected, compromised, and in a shared networking configuration can be used by an attacker to scan both the private and public address spaces. The other virtual machines on the host (if not patched) can also be exploited via the network, so a software firewall on each of the guests is recommended.
- (Enterprise version) When turning on shared folders, they can be accessed through a compromised guest. Files can then be placed on the host and attackers can access other guests' file systems.