What is a hypervisor and what security risks does virtualization introduce?

A hypervisor is software that creates and manages virtual machines by abstracting physical hardware resources. Type 1 hypervisors like VMware ESXi run directly on hardware, while Type 2 hypervisors like VirtualBox run on top of a host operating system. Security risks include VM escape, where an attacker breaks out of a virtual machine to access the hypervisor or other VMs; VM sprawl, where unmanaged virtual machines accumulate without proper patching; and side-channel attacks that exploit shared hardware resources.