Companion Malware

Companion Malware, aka companion viruses, use a feature of DOS that allows software programs with the same name, but with different extensions, to operate with different priorities. Most companion malware create a COM file which has a higher priority than an EXE file with the same name. A virus may detect that your computer has the file SOMEPROGRAM.EXE and create a file called SOMEPROGRAM.COM. If you execute "SOMEPROGRAM" from a DOS prompt command line the SOMEPROGRAM.COM file will be run before SOMEPROGRAM.EXE. Often the malware will then execute the original program, so everything appears normal.