Companion Virus

What Does Companion Virus Mean?

A companion virus is a complicated computer virus which, unlike traditional viruses, does not modify any files. Instead, it creates a copy of the file and places a different extension on it, usually .com. This unique quality makes a companion virus difficult to detect, as anti-virus software tends to use changes in files as clue.


The companion virus is an old type of virus that was more prominent during the MS-DOS era. It is propagated mostly through human intervention.

Techopedia Explains Companion Virus

When the user executes a program with the command prompt, he or she usually types in the name of the program. Because MS-DOS does not need a specification of the file type, it automatically runs the first file name that matches what the user types. So, if a companion virus copies file.exe and renames it, because comes before file.exe, MS-DOS will run that first program, thus spreading the infection in the computer, unbeknownst to the user.

Companion viruses mostly need human intervention to further infect a computer and with the advent of Windows XP, which does not use the MS-DOS interface much anymore, there were fewer ways for this type of virus to propagate itself. However, it can still work if a user double clicks it unintentionally or is run by accident, especially if the “show file extensions” option is not activated.


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Margaret Rouse

Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical, business audience. Over the past twenty years her explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…