Computer Worm Examples (2024): The 9 Worst Attacks Ever

By Tibor Moes / Updated: January 2024

Computer Worm Examples (2023): The 10 Worst Attacks Ever

Computer worms represent a significant cybersecurity threat, capable of causing widespread damage to digital infrastructures and personal data.

In this article, we will explore the nine most devastating computer worm attacks in history, providing insights into their impact and the lessons learned from each incident.


A computer worm is a type of malicious software program that replicates itself to spread to other computers, often causing harm by consuming bandwidth or corrupting data.

  • Morris Worm (1988): One of the first worms to gain widespread attention, it quickly infected a significant portion of the early internet’s computers. It caused damage estimated in the millions, affecting around 6,000 computers.
  • Melissa (1999): This worm spread via email, causing widespread disruption and necessitating costly cleanups. The total damage was estimated at around $80 million.
  • ILOVEYOU Worm (2000): Disguised as a love letter, this worm rapidly infected millions of Windows PCs worldwide. Over ten million computers were affected.
  • Code Red Worm (2001): Targeting Microsoft’s IIS web server software, it infected over 359,000 systems in less than 14 hours, causing over $2 billion in damages.
  • Slapper (2002): A worm targeting Linux systems, it plateaued at about 7,000 infected servers. It was notable for creating a network of compromised computers.
  • SQL Slammer (2003): This fast-spreading worm affected over 250,000 computers globally, impacting internet speeds and services. It highlighted vulnerabilities in database management systems.
  • Mydoom Worm (2004): Known for its rapid email-based propagation, Mydoom caused an estimated $38 billion in damages. It infected around 50 million computers worldwide.
  • Sasser (2004): Exploiting a Windows vulnerability, Sasser infected about 2 million computers. It caused frequent crashes and reboots, disrupting operations globally.
  • Stuxnet (2010): A sophisticated worm targeting industrial control systems, it ruined one-fifth of Iran’s nuclear centrifuges and degraded 1,000 machines. Over 200,000 computers were infected.

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Computer Worm Examples

1. Morris Worm (1988)

In the late 1980s, the digital world witnessed one of its first major security crises with the emergence of the Morris Worm. This seemingly innocuous piece of code, created by a Cornell University graduate student, Robert Tappan Morris, quickly spiraled out of control.

Within a mere 24 hours of its release, the worm had infiltrated an estimated 6,000 computers. This number might seem modest by today’s standards, but it was a significant percentage of the roughly 60,000 computers connected to the then-nascent Internet.

The financial repercussions were staggering. As reported by the FBI, initial damage assessments started at a hefty $100,000, but as the full extent of the worm’s impact became clear, these figures skyrocketed into the millions.

The Morris Worm was a wake-up call, highlighting the fragility of interconnected computer systems and the potential havoc that could be wreaked by a single piece of malicious code.

2. Melissa (1999)

Fast forward to 1999, and the digital landscape faced another formidable challenge with the Melissa virus. Named after an exotic dancer from Florida, this virus was the brainchild of David L. Smith.

Melissa masqueraded as an innocuous email attachment but, once opened, it replicated itself by sending messages to the top 50 contacts in the user’s Microsoft Outlook address book. This rapid multiplication caused a massive strain on email servers worldwide.

The FBI estimated the collective damage caused by the Melissa virus at around $80 million. This staggering sum was primarily attributed to the cleanup and repair of the affected computer systems. Melissa’s rampage served as a stark reminder of the vulnerabilities inherent in widely used software and the ease with which a well-crafted virus could disrupt global digital infrastructure.

3. ILOVEYOU Worm (2000)

The turn of the millennium saw the emergence of one of the most infamous computer worms in history: the ILOVEYOU worm. This deceptively named worm wreaked havoc on a global scale, exploiting human curiosity and trust.

Disguised as a love letter sent via email, the worm lured users into opening an attachment that unleashed its malicious payload. According to, the ILOVEYOU worm rapidly spread across the globe, infecting over ten million Windows personal computers starting from May 5, 2000.

The simplicity of its distribution method – an email from a known contact with an alluring subject line – played a key role in its widespread impact. The ILOVEYOU worm not only caused substantial data loss by overwriting files but also highlighted the vulnerabilities in email-based communication systems and the need for heightened awareness about digital security practices among individual users.

4. Code Red Worm (2001)

A year after the ILOVEYOU incident, the digital world faced another significant threat: the Code Red worm. This worm targeted computers running Microsoft’s IIS web server software, exploiting a buffer overflow vulnerability to replicate itself and spread across networks.

The speed and scale of its spread were alarming – as reported by researchers David Moore, Colleen Shannon, and Kimberly C. Claffy, Code Red infected over 359,000 systems in less than 14 hours. The worm’s rapid proliferation not only caused internet slowdowns but also compromised the security of affected systems.

The financial toll of the Code Red worm was immense, with the total damage estimated to be over $2 billion. This incident underscored the importance of timely software updates and patches, as well as the need for robust security measures in protecting critical internet infrastructure.

5. Slapper (2002)

In 2002, the digital world encountered a unique threat targeting Linux-based systems: the Slapper worm. Unlike its predecessors that primarily targeted Windows systems, Slapper exploited a vulnerability in the OpenSSL cryptographic software library, used by many servers running Linux. According to, the spread of this worm eventually plateaued at around 7,000 servers.

While this number may seem modest compared to other massive outbreaks, the significance of Slapper lay in its method of attack. It created a network of infected computers, known as a botnet, which could be used for coordinated attacks or to spread spam emails.

The Slapper worm was a wake-up call for the Linux community, emphasizing the need for constant vigilance and regular updates even in systems that were considered more secure than their Windows counterparts.

6. SQL Slammer (2003)

The following year, in 2003, the SQL Slammer worm emerged, causing widespread disruption. This worm exploited a vulnerability in Microsoft’s SQL Server and Desktop Engine database products. SQL Slammer was incredibly efficient in its design, allowing it to spread rapidly across the globe. reports that globally, over 250,000 computers were thought to have been affected by this worm.

The impact of SQL Slammer was not just in the number of infected systems but also in the collateral damage it caused. It significantly slowed down general internet traffic and even caused outages in some critical services, including ATM withdrawals and airline flight schedules.

The SQL Slammer incident highlighted the far-reaching consequences of cyberattacks on essential services and infrastructure, underscoring the critical need for secure coding practices and timely application of security patches.

7. Mydoom Worm (2004)

In 2004, the Mydoom worm emerged as one of the most damaging malware outbreaks in history. According to, Mydoom caused an astonishing $38 billion in damages, earning it the notorious title of one of the worst viruses ever.

This worm spread primarily through email, with messages containing deceptive subject lines to entice recipients into opening the attachment, thereby triggering the worm. Once activated, Mydoom replicated and sent itself to email addresses found in the user’s contact list, rapidly multiplying its reach.

Security researchers estimate that Mydoom infected around 50 million computers worldwide, showcasing its devastating efficiency.

The financial impact of Mydoom was felt across various sectors, from individual users to large corporations, highlighting the extensive vulnerabilities in email communication systems and the need for comprehensive security measures.

8. Sasser (2004)

Also in 2004, the digital world faced another significant threat from the Sasser worm. Unlike Mydoom, Sasser did not require user interaction to spread. Instead, it exploited a vulnerability in Microsoft Windows. Once a computer was infected, the worm scanned for other vulnerable systems and propagated itself.

According to, Sasser and its variants infected about 2 million computers worldwide. The worm caused computers to crash and reboot frequently, leading to significant disruptions in personal, business, and even critical public service operations.

The Sasser outbreak underscored the importance of regular system updates and the potential consequences of unpatched security vulnerabilities. It also highlighted the need for better awareness and preparedness against such threats, especially in critical infrastructure sectors.

9. Stuxnet (2010)

In 2010, the world witnessed a new era of cyber warfare with the discovery of Stuxnet, a highly sophisticated computer worm unlike any seen before. Stuxnet was not just a tool for data theft or system disruption; it was a weapon designed for physical sabotage.

According to M.A.C. Solution, Stuxnet had a devastating impact on Iran’s nuclear program, reportedly ruining almost one-fifth of the country’s nuclear centrifuges. The worm specifically targeted industrial control systems used in critical infrastructure, marking a significant shift in the nature of cyber threats.

Stuxnet infected over 200,000 computers, but its most alarming capability was its ability to cause physical damage. The worm made 1,000 machines physically degrade by manipulating the industrial control processes they were designed to safeguard.

This level of sophistication in Stuxnet’s design allowed it to remain undetected while it carried out its destructive tasks, demonstrating a new level of cyber threat that could bridge the gap between the digital and physical worlds.


The history of computer worms, from the Morris Worm in 1988 to Stuxnet in 2010, underscores a critical aspect of our digital world: the constant and evolving threat posed by malicious software.

These examples, each causing significant damage and disruption, highlight the importance of vigilance and proactive measures in cybersecurity. They serve as stark reminders of the potential vulnerabilities in our interconnected systems and the ongoing need to stay ahead of cyber threats.

In today’s digital age, where threats are ever-evolving and increasingly sophisticated, investing in robust antivirus software is more crucial than ever, especially for users of popular operating systems like Windows 11. Renowned brands like Norton, Avast, TotalAV, Bitdefender, McAfee, Panda, and Avira offer comprehensive protection that goes beyond basic defense mechanisms.

These antivirus solutions provide multi-layered security, including real-time threat detection, system vulnerability assessments, and advanced features like ransomware protection and identity theft safeguards.

By choosing a reliable antivirus program, users can significantly reduce the risk of falling victim to the next generation of computer worms and other cyber threats, ensuring their digital safety and the integrity of their personal and professional data.




Author: Tibor Moes

Author: Tibor Moes

Founder & Chief Editor at SoftwareLab

Tibor has tested 39 antivirus programs and 30 VPN services, and holds a Cybersecurity Graduate Certificate from Stanford University.

He uses Norton to protect his devices, CyberGhost for his privacy, and Dashlane for his passwords.

You can find him on LinkedIn or contact him here.