What is a Computer Worm?

Published On - March 20, 2024

Computer worms are malicious software that can autonomously replicate and spread across networks without human intervention. They leverage internet connections or local area networks to disseminate themselves to other computers. Worms differ from viruses and Trojan horses in their self-replicating nature, allowing them to spread rapidly. Unlike viruses, which require human activation to initiate the attack, worms can operate independently and transmit numerous copies of themselves, posing a significant threat. Trojan horses, on the other hand, disguise themselves as legitimate code and focus on compromising devices without attempting to propagate themselves through replication.

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Computer worms pose significant threats due to their ability to execute a wide array of attacks. Self-replication enables them to crash systems, while they can also stealthily download malicious applications and create backdoors for hackers to exploit. Additionally, worms are notoriously difficult to contain. Their rapid, automated spread across systems can make eradication a laborious and time-consuming process. For instance, in data storage environments, it may take months to thoroughly cleanse systems affected by a worm outbreak. Even worms without malicious payloads pose a major inconvenience for IT professionals, who must divert valuable resources to manage incident response.

Types of Computer Worms

Below are the most common types of computer worms:

Email Worms

Email worms, also known as mass-mailer worms, spread through email messages. These worms attach a copy of themselves to emails or provide links to infected files hosted on compromised websites. When an unsuspecting recipient opens the attachment or clicks the link, the worm infects their device.

File-Sharing Worms

File-sharing worms camouflage themselves within harmless-looking media files. When a user downloads the file, the worm infects their device. It then gains access to sensitive information on the device, which attackers can exploit for their own gain or sell to other cybercriminals.

IM Worms

IM worms disguise themselves as attachments or links shared on social media platforms. They often employ enticing content to lure the victim into clicking on the URL. Upon clicking, the worm spreads throughout the instant messaging network.


Cryptoworms encrypt data on the victim’s system, rendering it inaccessible. The worm then demands a ransom payment in exchange for providing a decryption key to unlock the data.

IRC Worms

IRC worms exploit Internet Relay Chat (IRC) channels to infect chat rooms and message forums. They do this by sending infected messages that spread the worm to other users on the network.

P2P Worms

P2P worms use peer-to-peer (P2P) networks to distribute copies of the worm to unsuspecting P2P users. They leverage the file-sharing capabilities of P2P networks to spread their infection. 

How Does a Computer Worm Work?

Computer worms typically exploit vulnerabilities within operating systems to infiltrate networks. These vulnerabilities may stem from backdoors and vulnerabilities in software, or even via USB flash drives. Once established within a network, worms grant cybercriminals access to perpetrate various nefarious activities, including:

  • Launching distributed denial of service attacks (DDoS) to overwhelm and disrupt networks.
  • Executing ransomware attacks that encrypt files and demand payment for their recovery.
  • Stealing sensitive data for financial or malicious purposes.
  • Introducing additional malware onto the network.
  • Deleting critical files.
  • Overloading the network, causing significant disruptions and downtime.  

Computer Worm Examples

For decades, computer worms have plagued companies across the globe. Below are some of the most destructive worms to date:

  • Morris (1988): MIT graduate Robert Morris released this worm, crippling over 6,000 UNIX machines and causing damages ranging from $100,000 to $10 million. It led to Morris’s felony conviction under the Computer Fraud and Abuse Act.
  • ILOVEYOU (2000): Also known as Love Bug, this worm spread via email attachments, infecting over 50 million PCs and causing approximately $15 billion in removal costs. 
  • SQL Slammer (2003): This worm targeted internet hosts, causing widespread denial of service, network congestion, and router failures. It infected 75,000 victim machines in just 10 minutes.
  • Mydoom (2004): One of the most rapidly spreading worms in history, Mydoom infected millions of Windows computers and resulted in an estimated $38 billion in damages. It remains a persistent threat, accounting for 1% of malicious emails.
  • Storm Worm (2007): Exploiting fears about a European weather disaster, the Storm Worm attacked millions of computers via email baiting.
  • Duqu (2011): A highly sophisticated worm allegedly linked to the creators of the Stuxnet worm, Duqu targeted industrial control systems.

Difference Between a Computer Worm, Virus and Trojan horse

Viruses, worms, and Trojan horses are distinct types of malware with varying methods of infection and impact. Viruses attach themselves to files or programs and spread through the distribution of those hosts. Worms, on the other hand, autonomously replicate and propagate from one computer to another, exploiting vulnerabilities to infect systems. In contrast, Trojan horses present themselves as legitimate programs but conceal malicious intentions. When executed, they infiltrate computers, allowing hackers to access the system via backdoors. Trojan horses can cause significant harm, making it essential to be vigilant against all forms of malware by employing robust security measures and maintaining a cautious approach when interacting with unknown files or programs. 

How to Find and Remove a Computer Worm?

Endpoint protection solutions are crucial for preventing worm infections, and analyzing their reports can confirm their effectiveness. However, if an endpoint protection solution fails to intercept a worm, there are certain signs that indicate its presence. These include:

  • Sluggish system performance due to high CPU usage.
  • Missing or concealed files and folders.
  • Unauthorized emails being sent.
  • Unexpected program crashes.
  • Unidentified files or programs.
  • Programs or websites launching without authorization.
  • Peculiar browser or program behavior.

In the event of a worm attack, it is essential to have a response plan in place to minimize its impact and speed up recovery. This plan should involve containing the attack to prevent its spread, assessing its scope by identifying affected systems, and eliminating all traces of the worm, such as removing malware, altering compromised accounts, and restoring affected systems.  

How to Prevent Computer Worms

To protect against the malicious impact of computer worms, organizations should prioritize preventative measures. Endpoint Detection and Response (EDR) systems will help you identify and mitigate worms and other cyber threats before they damage host computers. Additionally, employees should undergo thorough awareness training to recognize and avoid potential threats and avoid clicking on suspicious links or downloading malicious attachments. Using DNS filtering further controls web access by blocking harmful or unwanted content, preventing accidental exposure to malicious websites. Lastly, maintaining a stringent patch management process is crucial to address system vulnerabilities that worms may exploit.

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