What is a computer worm and why is it so dangerous?

Ray Fernandez

Sep 25, 20238 min read

What is a computer worm and why is it so dangerous? Header image

Imagine a malware so self-sufficient that it can clone itself, infect computers without interacting with the victim, and scan through online networks seeking out other devices to infect. It may sound like something out of a movie, but this threat is real. It’s known as a worm malware.

Computer worm: The definition and how it works

What is a worm? In cybersecurity terms, a worm is a type of malicious software (malware) that can replicate itself and spread across computers and devices, usually through networks, leaving an active copy of itself in each device it crosses paths with.

Computer worms were first created in 1987 and still exist today. An estimated one million worms are actively causing havoc. Hackers and cybercriminals still turn to them because they are coded to be completely undetectable and are among the most destructive malware.

How computer worms work

Worms scan internet networks in search of computers, smartphones, and tablets with specific vulnerabilities they can exploit. The worms then use these vulnerabilities, usually found in operating systems, as doors into computers, mobile devices, and tablets.

Once a worm has set foot inside a computer or device, it will self-replicate and continue moving on. The copy it leaves behind is then activated. And depending on what it was coded to do, it may steal sensitive data, change system settings, send information, launch bulk messaging or email campaigns, and more. The process of the worm cloning itself, spreading, and launching attacks is instant.

What makes worms even more dangerous is that they require no action on the part of the victim. Contrary to viruses or other malware, which must trick users into taking a specific action (clicking on a link or downloading a file), worms are able to spread independently. Worms also work in stealth mode in the background. 

How do worms spread? In the old days, hackers would infect computers with worms by gaining physical access. This meant they had to get up close to the computer and insert a floppy disc or other types of media into the computer’s drive. But today, worms spread using different techniques. 

The most popular vectors of attack of worms are:

  • Email (phishing or spam)
  • SMS messages
  • Through network scans
  • Messaging apps like WhatsApp or Messenger
  • File sharing sites
  • External media devices include USB, memory cards, and external hard drives

Frequently asked questions about computer worms

Do worms infect mobile phones?

Yes. Worms can infect all types of smartphones. Some, known as mobile worms, are specifically designed to target mobile devices. These can breach a device through a malicious file, email, SMS, messaging app, or network.

Can a worm spread to a USB device?

Many worms still spread today through USB media. Cybercriminals may insert the worm into the USB, or a worm on a computer may infect a clean USB. Because worms are self-contained, USB spreading is automatic and requires no action from the victim.

How fast do computer worms spread?

A worm can go global in just minutes, spread, and infect hundreds of thousands of computers, including devices at high-level organizations such as governments with cutting-edge cyber security technology.

As previously mentioned, worms are designed to instantly infect, spread, and activate. The speed at which a worm can spread will depend on how efficiently coded it is and the resources and power of infected computers and devices. Worm attacks can last a couple of hours before experts find the kill switch.

Can a computer worm carry a virus?

Yes, a worm can contain a virus or other malware, such as ransomware or spyware. Worms can also open backdoors for malware to be easily installed on a computer or device.

Symptoms of a computer worm

While worms are very good at hiding, there are several symptoms you may experience that should immediately raise red flags.

Worm symptoms include:

  • Your computer is slow or crashing: Worms feed off the resources of your device, such as memory, RAM, and hard drive space, and are very active and aggressive. This activity tends to slow down computers and smartphones, make them act up or behave strangely, and even crash.
  • You are out of storage space: If you get warnings that your smartphone or personal computer is running out of space, this may be a sign of a worm. Worms take up hard drive space when they replicate.
  • Your email, SMS, or messaging service has suspicious activity: Many worms launch messaging campaigns in bulk to continue their path, infecting your contacts and other users. Worms will hijack your email account, SMS app, and online messaging services. If you get warnings and notifications, notice messages you didn’t send, or learn about contacts receiving strange messages from you, look into the issue immediately.

The difference between a computer worm and other malware

Worms are a unique kind of malware. They can do many things that other malicious software can’t do, such as self-replicate independently. They also have other impressive tricks under their sleeves, such as speed and the ability to search for other devices. Let’s look at the differences between worms and other types of malware.

Worm vs. Trojan

The main difference between a Trojan and a worm is how they infect a device. Trojans get their name from the hollow wooden horse used by the Ancient Greeks used to breach the walls of Troy by hiding soldiers inside during the Trojan War. A Trojan malware behaves in the same way. It appears to its victims as a piece of legitimate software but contains malware hidden within it. Worms do not use this technique.

Another difference is that Trojans usually require that users download and install a program. At the same time, worms never present themselves to users but instead breach devices undetected, requiring no action from the victim. Trojans do not have the self-replicating ability that worms have, but they can both open backdoors and spread through networks.

Worm vs. virus

Viruses, like Trojans, require human action to initiate. This means a virus can only begin causing damage when a user downloads and runs the program. While both viruses and worms can replicate, they do so differently.

Like a worm, a virus can spread from one computer to another, leaving infections behind as it travels, but every time it moves, it needs to trick the new user into installing it. Worms don’t need the victim to take any action.

What is the most famous computer worm?

The two worms that come to mind when thinking about the most infamous worms in cybersecurity history are WannaCry and Stuxnet.

The 2017 WannaCry ransomware cryptoworm infected 230,000 computers in 150 countries in just a couple of hours, sending shockwaves across businesses, politics, government, organizations, and the cyber security industry.

On the other hand, Stuxnet stands as the most sophisticated worm ever created. It was discovered in 2010, but experts believe it has operated since 2005. Stuxnet is more than a worm; it is a cyberweapon. It was developed under an operation run by the United States and Israel under the code name: Operation Olympic Games.

Stuxnet is believed to be responsible for causing damage to Iran’s nuclear program. The worm is not only digitally capable but can even control industrial machinery using infected hosts. Estimates say the worm infected more than 200,000 computers and caused damage or disruption to over 1,000 industrial operations.

Other notorious examples of computer worms

Many other computer worms have jumped to fame due to their attacks. Here are some of the most notorious:

The Morris worm or Internet worm: Released on November 2, 1988, this worm exploited weak passwords to infect thousands of computers in just 10 minutes. Estimates place the economic impact of the attack as high as $10 million.

The Bagle worm: Also known as the Beagle, Mitglieder, or Lodeight, was launched on January 18, 2004. The mass mailer worm malware initially infected 120,000 computers. The Bagle worm led to several variants. The Bagle botnet comprised an estimated 150,000-230,000 computers infected with the Bagle worm.

Conficker (AKA Downup, Downadup, or Kido): This worm exploited flaws in the Windows operating system and infected millions of computers in over a hundred countries.

SQL Slammer: The 2003 SQL Slammer, a brute-force worm, spread at lightning speed, infecting about 75,000 victims in just 10 minutes.

ILOVEYOU: This worm infected over ten million Windows personal computers when it launched in May 2000. It spread as an email message with a subject line that read “ILOVEYOU.” The worm preyed on a Windows vulnerability and used attached malware worm files.

How to prevent your Mac from getting infected with a worm

Worms sound unstoppable and extremely dangerous. But you can take simple steps to prevent computer worms from infecting your Mac.

Get a trusted and effective antimalware

Good antimalware software will detect, isolate, and remove worms running simple scans. In fact, the best protection against worms is antimalware that can run live scans, remain updated on new threats, isolate damaging executing code, and completely remove worms and all traces of them from your system. 

To help keep your Mac safe, check out CleanMyMac X. Its Malware Removal module, powered by Moonlock Engine, will protect your Mac from worms and keep your computer in top shape. 

The Malware Removal module in CleanMyMac X, powered by Moonlock Engine

Be aware of suspicious messages

While worms can breach your computer through networks, many attackers still use email, SMS, messaging, social media, malicious websites, and other social engineering techniques to trick you into clicking links or downloading files. Never open attachments from unknown contacts you don’t trust. Run antimalware on your communication channels, and be vigilant of strange messages.

Keep your firewalls up!

Firewalls are critical to keeping data secure. Fortunately, Macs have built-in firewalls that can be activated with just a few clicks. Combined with antimalware and firewalls, they are the most effective resources to stop worm attacks before they happen.

To enable your Mac’s firewall (macOS Ventura): 

  1. Choose the Apple menu and select System Preferences.
  2. Now click Network and select Firewall. (For macOS Monterey and Big Sur, click Preferences, then Security & Privacy, then Firewall).
  3. Turn Firewall on.
  4. Click on Options for additional security settings (for example, allow only specified or essential apps and services, or turn on “Enable stealth mode”). 

Update your operating system, software, and apps

Worms that use internet networks to spread exploit vulnerabilities found in operating systems, software, programs, and web and mobile applications. Apple developers are constantly fighting against cybercriminals. That said, hackers will always find new ways to bypass existing Mac security features, and Apple cybersecurity experts will patch those vulnerabilities. Updates often include security patches, which are essential to stop the threats that are currently out there. Make sure you keep your Mac system fully updated.

The history of computer worms is fascinating, but its present and future states are concerning. Worms will continue evolving and are poised to become more dangerous every day. Understanding how a worm works, its symptoms, and what actions you can take to prevent them is critical. Stay informed and keep your guard up.

Ray Fernandez Ray Fernandez
Ray has been covering tech and cybersecurity for over 15 years. His work has appeared on TechRepublic, VentureBeat, Forbes, Entrepreneur, and the Microsoft Blog, among others.