A polymorphic engine (sometimes called mutation engine or mutating engine) is a software component that uses polymorphic code to alter the payload while preserving the same functionality.
One common deployment is a file binder that weaves malware into normal files, such as office documents. Since this type of malware is usually polymorphic, it is also known as a polymorphic packer.
The engine of the Virutbotnet is an example of a polymorphic engine.[1]