When security researchers talk about malware, they usually refer to files stored on a computer system, which intends to damage a device or steal sensitive data from it. Those files can be scanned by AV engines and can be handled in a classic way. The following analysis is an example of malware which resides in the registry only, is persistent and is not present as a file which can be scanned easily.
more here..............https://blog.gdatasoftware.com/blog/article/poweliks-the-persistent-malware-without-a-file.html
more here..............https://blog.gdatasoftware.com/blog/article/poweliks-the-persistent-malware-without-a-file.html