Companion Virus

Definition of Companion Virus

A companion virus is a type of computer virus that attaches itself to legitimate programs and runs simultaneously with them. It doesn’t modify the original program’s file; instead, it creates a separate infected file with a similar name, leading the user to execute the virus unintentionally. As a result, the virus spreads and can cause damage or allow unauthorized access to the infected system.


The phonetics for the keyword “Companion Virus” using the International Phonetic Alphabet (IPA) would be:/kəmˈpæn.jən ˈvaɪ.rəs/

Key Takeaways

  1. Companion Virus is a type of malware that disguises itself as a legitimate program and is designed to run parallel to an existing executable file without altering or infecting it directly.
  2. It typically spreads by attaching itself to other applications or files and is executed when the user inadvertently launches the infected program, leading to the simultaneous launch of the virus.
  3. Preventing Companion Virus infections requires adopting good security practices like using a reputable antivirus program, updating software regularly, and avoiding the download of files from unknown sources.

Importance of Companion Virus

The term “Companion Virus” is important in technology as it highlights a specific type of computer virus that quietly coexists with legitimate files without modifying them.

By creating files with similar names but different extensions, they can exploit operating systems’ file execution hierarchy, thus intercepting and executing before the intended application.

This deceptive approach allows the virus to spread and cause damage while remaining undetected by traditional antivirus software and the user.

Furthermore, the existence of companion viruses underscores the need for robust cybersecurity measures and improved threat detection methods to counteract such malware and protect valuable data and systems.


A companion virus is a unique type of computer virus that attaches itself not by directly infecting executable files, but rather by accompanying them with a separate malicious file. The primary purpose of a companion virus is to deceive users and the system by taking advantage of the file execution process to propagate and inflict harm.

This type of virus proves to be particularly sneaky as it leaves no traces on any existing files while it continues to spread and execute its payload. As companion viruses possess the ability to execute their malicious code before the legitimate file is executed, they can effectively disrupt or manipulate the latter’s actions, potentially causing damage to data or system functionalities.

Companion viruses are often used for a variety of nefarious purposes which include data theft, unauthorized access to a system, disruption of normal system operations, and corruption of vital information. Among other tasks, some companion viruses can also deliver spyware or adware programs that are designed to monitor a user’s online behaviors and preferences.

Since these viruses exist as separate but complementary files to the legitimate ones, traditional security software may struggle to recognize and remove them. As a result, it is crucial for individuals and enterprises alike to adopt a proactive, multi-layered approach to cybersecurity, investing in robust antivirus protection, and prioritizing user education about the prevalent threats and best practices to avoid them.

Examples of Companion Virus

A companion virus is a type of computer virus that attaches itself to a legitimate program and runs when the infected program is executed. They typically have the same file name as the target program but use a different file extension. Though most modern antivirus software can effectively deal with companion viruses, here are three historical examples of companion viruses:

Stator Virus (also known as the Asimov Virus): First discovered in the 1990s, the Stator Virus was a unique companion virus that infected executable files (.EXE) by creating a duplicate with the same name but a different file extension (.COM). When a user attempted to run the infected program, the operating system would execute the .COM file instead. The Stator Virus caused no direct harm to the infected system, but it spread quickly.

Ghostball Virus: Ghostball was a well-known companion virus from the late 1980s and early 1990s. It created a new .COM file for each .EXE file, with the virus code loaded into memory upon execution of the .COM file. Ghostball would then let the original .EXE file run while spreading the virus to other .EXE files. The virus often displayed a light show effects sequence on the computer screen.

Frodo Virus: Discovered in 1989, the Frodo Virus was a stealthy companion virus. It infected .COM and .EXE files, generating a secondary .COM file with the same name and a hidden attribute on the infected file. The Frodo Virus piggybacked on program execution to load itself into memory, then reproduced quietly without giving any indication of its existence. It was noted for its destructive capability of overwriting disk sectors and rendering systems inoperable.


FAQ for Companion Virus

1. What is a Companion Virus?

A Companion Virus is a type of computer virus that infects a computer system by creating a file with a different filename extension while keeping the same file name as an existing legitimate program. When the user unknowingly executes the infected file, the virus spreads and may potentially cause damage to the computer system.

2. How does a Companion Virus spread?

A Companion Virus spreads when a user executes the infected file, thinking it is the legitimate program they intend to run. The virus can then spread to other programs and computer systems through file sharing, email attachments, or downloading from the internet.

3. What are the signs of a Companion Virus infection on a computer system?

Signs of a Companion Virus infection may include unexplained changes in file sizes, higher CPU usage, unusual error messages, frequent system crashes, and an increase in files with unknown extensions or duplicate file names.

4. How can I protect my computer system from Companion Viruses?

To protect your computer system from Companion Viruses, always use a reputable antivirus software, keep your system and software up-to-date, avoid downloading files from unknown sources or opening suspicious email attachments, and regularly scan your computer for malware.

5. How can I remove a Companion Virus from my computer system?

To remove a Companion Virus from your computer system, first, disconnect your system from the internet to prevent further infection. Next, run a thorough system scan using a reputable antivirus software, which will identify and quarantine the infected files. Finally, either follow the antivirus software’s instructions to remove the virus or restore the system to a previous, uninfected state using a backup.


Related Technology Terms

  • Malware
  • Anti-Virus Software
  • File Infection
  • Trojan Horse
  • Worm

Sources for More Information


About The Authors

The DevX Technology Glossary is reviewed by technology experts and writers from our community. Terms and definitions continue to go under updates to stay relevant and up-to-date. These experts help us maintain the almost 10,000+ technology terms on DevX. Our reviewers have a strong technical background in software development, engineering, and startup businesses. They are experts with real-world experience working in the tech industry and academia.

See our full expert review panel.

These experts include:


About Our Editorial Process

At DevX, we’re dedicated to tech entrepreneurship. Our team closely follows industry shifts, new products, AI breakthroughs, technology trends, and funding announcements. Articles undergo thorough editing to ensure accuracy and clarity, reflecting DevX’s style and supporting entrepreneurs in the tech sphere.

See our full editorial policy.

More Technology Terms

Technology Glossary

Table of Contents