What Is a Computer Worm? Types, Examples & How to Protect Your PC

Worm malware is one of the oldest and most stubborn threats in the cybersecurity world. Unlike other harmful software that requires users to click or download something, worms are sneaky. They can copy themselves and hop from one computer to another, usually without anyone noticing. One worm can overload networks, mess up services, and put sensitive information at risk.

The danger is definitely out there. According to Kaspersky’s 2025 ICS Threat Report, worms appeared in thousands of industrial systems globally during the first part of the year, and infection rates started climbing as the year went on. This shows that worms are still a go-to tool for cybercriminals and can seriously threaten individuals and businesses.

In this guide, you’ll find out what a worm is, how it spreads, differs from a virus, and what signs might indicate an infection. There are also examples of some major worm attacks. Plus, you’ll learn to detect worms and some strategies to keep them at bay to boost your defenses against this ongoing threat.

What Is a Computer Worm?

A computer worm is malicious software that can replicate itself and spread to other computers without user action. Unlike viruses, which require user interaction, worms exploit security flaws to hop from device to device, making them particularly dangerous due to their rapid multiplication.

Worms operate independently, scanning networks for insecure devices and spreading continuously. Some simply consume bandwidth and processing power, while others can steal data or install ransomware.

Today, worms can adapt to various environments and may spread through phishing emails, USB drives, or vulnerabilities in network protocols, posing a persistent threat to individual users and businesses.

Is a Worm a Virus?

Worms and viruses are both types of malware, but they function differently. A virus attaches itself to files, programs, or operating systems and activates when a user opens the infected item. Without user interaction, a virus cannot spread.

Conversely, a worm operates independently by scanning networks for vulnerabilities and replicating itself, allowing it to spread rapidly across systems.

While both can cause harm, distinguishing between them is essential for protection. Removing infected files is key to viruses, whereas stopping worms involves monitoring networks and managing software updates.

How Computer Worms Operate?

Computer worms aim to spread quickly and widely. They do this by taking advantage of system weaknesses and poor network security. Knowing how they infect systems in different steps helps us understand why worms remain a significant cybersecurity threat.

Step 1: Identifying Vulnerabilities

The infection begins with the worm scanning networks for weaknesses. These weaknesses might be unpatched operating systems, misconfigured services, or outdated applications. Once a target is identified, the worm leverages the vulnerability as an entry point. Worms often use automated scanning tools to probe thousands of systems within minutes.

Step 2: Initial Exploit and Execution

After finding a vulnerable system, the worm exploits it to gain access. This can involve buffer overflow attacks, SQL injection, or other code execution flaws. Once inside, the worm executes its payload, embedding itself into the compromised device without requiring the user to click or install anything.

Step 3: Establishing Persistence

The worm then sets mechanisms to maintain its presence. This might involve creating scheduled tasks, modifying system registries, or injecting itself into running processes. Persistence ensures the worm continues operating even after reboots or attempts to remove it.

Step 4: Replication and Network Propagation

Once stable on the host system, the worm replicates its code and starts looking outward. It actively scans the local network, the wider internet, or connected devices for more targets. Using the same exploit it leveraged on the first system, it deploys copies of itself to newly compromised machines, multiplying its presence rapidly.

Step 5: Payload Delivery

While some worms are designed to spread and consume resources, many deliver additional payloads. These payloads may include spyware, ransomware, or backdoors that allow attackers remote access. The worm creates the bridge, after which other malware can execute specific malicious objectives.

Step 6: Continuous Reinfection and Evolution

Modern worms are not static. Some are coded to evolve by downloading updated instructions or modules from command and control servers. Others reinfect already compromised devices to ensure dominance or update their payload with new capabilities like Polymorphic and metamorphic worms, which evolve by altering their code to evade detection. This makes containment more difficult and increases the long-term impact.

Signs Your Computer Has a Worm

Unlike many malware that try to remain hidden, computer worms often leave noticeable traces of their activity. Because they spread aggressively and replicate without user action, their presence can quickly disrupt a device or network’s operation. Identifying these red flags early can help prevent severe damage.

• Worms consume processing power and memory, making even simple tasks like opening files or browsing the internet noticeably sluggish.
• Your device may become unstable as worms interfere with system processes or overload resources.
• Constant data uploads or downloads in the background can signal that the worm is spreading or stealing information.
• Worms often create new files, duplicate existing ones, or install suspicious applications without your permission.
• If your antivirus or firewall turns off unexpectedly, it can strongly indicate that a worm is trying to avoid detection.
• Worms may alter browser settings, leading to persistent pop-ups or being redirected to unfamiliar websites.
• If your contacts report receiving suspicious links or attachments from you, the worm could use your account to spread.

How to Prevent and Mitigate Worm Attacks

Worms can spread fast and quietly, usually taking advantage of weak systems or insecure networks. The best way to defend yourself is to prevent them, but it’s also super important to have a plan for when they strike. Here are some easy ways to keep yourself safe and lower your risks:

Keep Systems and Software Updated

Most worms exploit vulnerabilities in outdated operating systems, browsers, and applications. Enabling automatic updates ensures that critical patches are installed as soon as they’re released, closing the loopholes worms often exploit. Businesses should also maintain a patch management system to streamline updates across all endpoints.

Use Security Tools as a First Line of Defense

A layered defense is essential. Antivirus and anti-malware solutions detect and quarantine worms before they cause damage, while firewalls control incoming and outgoing traffic to block suspicious connections. At an enterprise level, intrusion detection and prevention systems (IDS/IPS) add another layer of protection against large-scale worm outbreaks.

Secure Network Connections with a VPN

Public Wi-Fi is a hotspot for worm distribution because of its weak or nonexistent encryption. Using AstrillVPN encrypts your internet traffic and conceals your IP address, making it much harder for worms to exploit network-level vulnerabilities. A VPN adds a vital shield for remote workers and travelers when connecting from airports, cafes, or hotels.

Practice Safe Email and Download Habits

Email is one of the most common ways for viruses to spread. Do not open attachments or click on links from people you don’t know. Only download apps, drivers, and updates from official websites or trusted stores. A harmful download can let a virus in and allow it to spread.

Segment and Harden Your Network

Network segmentation helps limit how far a computer worm can spread in your system. By isolating sensitive systems and using access controls, you can contain infections and reduce their impact. Strong authentication methods and strict permissions will further decrease your risk of exposure.

Monitor and Analyze Network Traffic

Unusual bandwidth spikes, unauthorized connections, or repeated login attempts may signal worm activity. Network monitoring tools, SIEM (Security Information and Event Management) systems, and anomaly detection software help identify suspicious behavior early, allowing faster response.

Back Up Data Regularly and Securely

No system is 100% immune, even with strong defenses. Regular backups ensure that you can restore systems quickly after an infection. Store backups in offline devices or secure cloud storage protected by encryption and MFA (Multi-Factor Authentication).

Close Unnecessary Ports and Services

Every open port is a potential doorway for a worm. Conduct regular network scans to identify unused services and disable them. By reducing the attack surface, you give worms fewer opportunities to spread.

Prominent Tools for Detecting Worms

Organizations and individuals rely on specialized security tools to effectively detect computer worms. Here are some of the most reliable ones:

1. Antivirus and Antimalware Software

Tools like Bitdefender, Kaspersky, and Malwarebytes scan systems for known worm signatures and suspicious behavior. They act as the first line of defense, alerting users to potential infections.

2. Intrusion Detection Systems (IDS)

Solutions such as Snort and Suricata monitor network traffic in real time, spotting unusual patterns that may indicate worm propagation. IDS tools are widely used in enterprise environments.

3. Endpoint Detection and Response (EDR) Tools

Platforms like CrowdStrike Falcon and SentinelOne provide advanced behavioral analysis, detecting worms that evade traditional antivirus. They also help isolate infected devices to prevent spread.

4. Network Scanners

Utilities like Nmap or enterprise-grade scanners identify unusual open ports and vulnerabilities that worms may exploit. Regular scanning helps administrators catch weak points early.

5. Security Information and Event Management (SIEM) Systems

SIEM tools like Splunk and IBM QRadar collect logs from across an organization’s systems. By analyzing anomalies, they can quickly detect large-scale worm activity.

6. Sandboxing Tools

Solutions like Cuckoo Sandbox let security teams safely execute suspicious files in an isolated environment. This helps confirm whether a file carries a worm before reaching the central system.

7. VPNs with Threat Protection

Some VPNs, like AstrillVPN, offer built-in threat protection features that block access to malicious sites hosting worms. They also encrypt traffic, making it harder for worms to spread across unsecured networks.

Examples of Computer Worms

Here are some well-known examples of computer worms you can include in your content, each with a short description of what made it significant:

1. Morris Worm

Often called the first major internet worm, it spread rapidly across Unix systems and caused massive slowdowns. It highlighted the risks of networked computing.

2. ILOVEYOU Worm

Disguised as a love letter email attachment, it spread worldwide in hours, infecting millions of Windows PCs and causing billions in damages.

3. Code Red Worm

Exploited a flaw in Microsoft IIS servers. It defaced websites and launched denial-of-service attacks against specific targets.

4. SQL Slammer

Spread within minutes by exploiting Microsoft SQL Server. It caused severe internet slowdowns globally and took down banking and airline networks.

5. Conficker

One of the most widespread worms, it infected millions of Windows machines, creating a large botnet used for criminal activities.

6. Stuxnet

A highly sophisticated worm believed to be state-sponsored. It targeted industrial control systems, specifically Iranian nuclear centrifuges, making it one of the first cyber weapons.

7. WannaCry 

A ransomware worm exploiting a Windows SMB vulnerability (EternalBlue). It spread rapidly across the globe, disrupting healthcare, telecom, and transport systems.

Major Worm Attacks in the Recent Past

Over the years, worm attacks have caused some of the most damaging cyber incidents in history. While their methods have evolved, the scale of destruction they leave behind is still significant.

CMoon USB Worm (2024)

This worm surfaced in mid-2024 and specifically targeted users in Russia by hiding inside a compromised gas company’s website. The infection was delivered as a disguised document. Once a user opened it, CMoon replicated through connected USB drives, stole credentials, took screenshots, and even launched DDoS attacks. It demonstrates how even targeted, localized worm campaigns can quickly spiral out of control. 

P2Pinfect Worm Targeting Redis Servers (2024)

Initially identified in 2023, this worm attacks publicly exposed Redis instances. By mid-2024, it was updated to deliver ransomware, cryptocurrency mining payloads, and self-replication. This shows how fast-moving worm threats adapt to maximize damage.

“This increase in P2PInfect traffic has coincided with a growing number of variants seen in the wild, suggesting that the malware’s developers are operating at an extremely high development cadence.”
— Matt Muir, Cado Security researcher

HIUPAN and PUBLOAD (2024)

Emerging in targeted campaigns, the HIUPAN worm spread the PUBLOAD malware via removable drives across Asia-Pacific government entities. The threat group Earth Preta adapted worm-based strategies with multi-stage infection tactics.

Mekotio and Other Banking Trojans with Worm-like Behavior (2020–2021)

Cybercriminals began combining worm-like propagation with banking malware to target financial data. Attacks in Latin America spread quickly through phishing and unpatched systems, showing that worms were being adapted into new cybercrime models.

EternalRocks (2022)

Discovered after the WannaCry incident, EternalRocks gained attention in 2022 for using multiple NSA exploits at once, making it far more dangerous. Unlike WannaCry, it did not include ransomware but focused on building a resilient botnet.

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