Many companies have gone full stream ahead with virtualization and have saved money by doing so. But some are beginning to worry now about the security of their virtual infrastructures. When servers were operating systems tied to one physical machine, it was easier to keep track of exactly what you had. Now that that a single physical machine can hold multiple virtual servers, your hardware inventory doesn't necessarily tell the whole story.
Hypervisors, virtual switches and other elements of a virtualized environment must be secured just like physical servers, and not everyone has applied best practices to the virtual parts of their networks. In this article from ComputerWorld, The Scary Side of Virtualization, the managing director of security research at the Info Pro says that "the questions about security in a virtual environment are centered around lack of visibility, lack of control and fear of the unknown." It's a long piece, but well worth the read if your company runs virtual machines - as most companies today do.