A virtual machine is an isolated software computer that runs inside a host system. It behaves like its own operating system and hardware, but it is created and controlled by virtualization software. In security, VMs are used to separate risky code, test malware, or run desktop agents without giving them direct access to the host.
VMs matter because they are often treated as a boundary: if a process is confined to the guest, it should not be able to read host files or control host services. Attacks that break this boundary are called sandbox escapes or VM escapes. They usually target weaknesses in file sharing, device emulation, mounts, or hypervisor integration. Defenders rely on tight permissions, minimal shared folders, and strong isolation, because a VM is only a defense when the host-guest boundary is enforced correctly.