What is a .COM File? Not Just Another Dotcom Bubble

Published: 9/7/2025

A .COM file is a simple, old-school type of executable file used by DOS and Windows operating systems. When you double-click a .COM file, it runs commands or programs directly on your computer.

Overview

Ever seen a file named with a .com extension and wondered, “Wait, is this just an ancient website?” Think again! While “dotcom” makes most folks think of internet gold rushes and flip phones, in the cybersecurity world, a .COM file is anything but nostalgic. Here's your one-stop glossary entry for the .COM file, what it does, how it’s different from .EXE files, and the real talk on whether it’s friend or foe in 2024.

What is a .COM File?

A .COM file is a type of executable file format mainly found in early DOS (Disk Operating System) and Windows systems. Unlike web addresses, .COM files don’t take you online—instead, they run code directly on your local machine.

Think of a .COM file like the Minions of the computer world: small, efficient, and foolishly eager to follow directions (for better or worse). They're a relic from 1970s computing, but their simplicity means even modern threats might try to use them as sneaky digital Trojan horses.

What does a .COM file do?

When opened, a .COM file instructs your PC to perform a set of actions. These can range from basic system tasks (like displaying a message or launching a utility) to, unfortunately, running malicious code. Back in the MS-DOS days, nearly every tiny utility—from calculators to games—came as a .COM file. Today, legit ones are rare outside vintage computing circles. Malware authors, however, still find them handy because their structure is easy to manipulate.

.COM vs .EXE files

Is a .COM file the same as a .EXE file?

Nope! Even though both.COM and .EXE files are executable (meaning you can run them), they’re built differently under the hood. Here’s the quick-and-dirty breakdown:

Major differences

Structure

.COM files are flat, raw binary files with no headers, sections, or real structure. They max out at 64KB in size.
.EXE files are fancier, with headers and sections for code, resources, and more. They can be much larger and more complex.

What runs first

.COM files just run from the first byte. There’s no prep or checking.
.EXE files start at a defined “entry point,” and have built-in error handling.

Compatibility

.COM files were created for MS-DOS and early Windows systems.
.EXE is still the ace for Windows applications today.

Security

Modern Windows systems give .EXE files more security attention due to wider use, but .COM files can still slip through less-protected cracks.

Technical pop quiz moment

If you’re stuck with both MYAPP.COM and MYAPP.EXE in the same folder, which is run when you type "MYAPP" in DOS? The .COM file wins the race every time!

Are .COM files still used today?

Not much. Most modern Windows applications use .EXE, .DLL, or other more feature-rich formats, leaving.COM files in the software graveyard. But just when you think they’re extinct, a retro hacker (or a bad guy) can bring them back for mischief or nostalgia.

Rare as they are, cybersecurity pros need to know about .COM files because malware sometimes uses them to get past modern defenses, especially if older or poorly-configured systems are in play. The FBI has documented malware campaigns that used .COM files as a secondary payload (see here).

Are these files safe?

By themselves, .COM files are neutral. They can be helpful (think retro games or tools), but their simplicity means they’re also prime targets for threat actors. Just like .EXE files, if someone sends you a random .COM file, don't open it unless you're sure of its source.

Can a .COM file be a virus?

Absolutely. Throughout the 1980s and '90s, .COM files were some of the most popular carriers for early computer viruses like the infamous "Cascade" and "Stoned." And yes, even today, threat actors can use .COM files to slip malware past less-savvy users or outdated antivirus programs.

How do I know if a .COM file is malicious?

Use these security best practices:

Only open .COM files from sources you trust.
Scan the file with up-to-date antivirus software.
Check its digital signature (if any).
Examine the file’s behavior in a safe, virtual environment.
If in doubt, upload to VirusTotal for a scan across multiple antivirus engines.

Remember, when it comes to weird or unexpected files, being paranoid isn’t just healthy, it’s job security.

Key Takeaways

.COM files are basic, old-format executables from the DOS/Windows era. They’re different from .EXE files in structure, size, and features. These files are mostly obsolete, but malware may still use them to bypass some defenses. Always scan or sandbox any unknown .COM file before running it.

Stay sharp and don’t fall for ancient tricks wearing digital disguises! When in doubt, double-check those file types before you execute anything “nostalgic.”

Frequently Asked Questions

A .COM file is a simple executable file that runs code on Windows or DOS systems. It tells your computer to perform specific actions when opened.

No, .COM files are simpler and smaller, while .EXE files are more complex and feature-rich.

Yes, if it contains malicious code. Treat unknown .COM files are just like any other executable, and scan them before opening.

They're rare but still pop up, especially in malware, research, and sometimes in vintage or embedded systems.

Scan with quality antivirus software, check the source, and consider tools like VirusTotal. For high-risk environments, analyze in a virtual machine.

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What is a .COM File? Not Just Another Dotcom Bubble