Understanding Worms and Trojans: How they work and how to protect your devices from malware

Cyber threats are expanding at an increasingly rapid rate compared to ever before, targeting businesses and individuals worldwide.

According to a 2024 report, malware attacks increased by 15% worldwide, marking one of the sharpest year-over-year rises in the past decade. Worldwide cybercrime costs are anticipated to reach $10.5 trillion annually by 2025.

Two of the most disruptive forms of malware are trojans and worms, which sound suspiciously similar but work quite differently. Over 560,000 new pieces of malware, including worms and trojans, are detected daily.

This guide explains worms and trojans, how they differ, and the best ways to protect your devices from these common malware threats.

What are computer worms, and how do they spread?

A worm is a type of malware that can replicate itself and spread to other computers and networks without user intervention.

It exploits operating system, software, or network connection vulnerabilities to quickly spread to as many devices as possible, often without anyone even being aware.

IBM’s 2023 Threat Intelligence Index reported that 17% of enterprise malware infections originated from self-propagating worms.

Key characteristics of worms:

• Self-replication: Unlike viruses, worms do not require a host file. They will replicate and propagate on their own.
• Rapid proliferation: Worms will infect tens of thousands of systems within a few hours, moving along networks at light speed.
• Minimal or no human interaction: A worm typically spreads without users having to click or download anything.
• Network disruption: Worms consume enormous bandwidth and system resources, slowing down networks and sometimes even crashing entire systems.

How worms spread:

• By exploiting out-of-date software or unpatched vulnerabilities.
• Through phishing emails with malicious links.
• Via infected USB drives or removable media.
• Across poorly secured networks.

Real-world example: The ILOVEYOU worm

One of the worst worm attacks occurred in May 2000, when the ILOVEYOU worm was released. Disguised as an email attachment of a love letter, it infected over 50 million systems within 10 days and was estimated to have inflicted $10 billion in damage.

Interestingly, ILOVEYOU demonstrated that worms could manipulate human emotions (social engineering) while exploiting technical weaknesses, a mix still present today.

What are Trojan malware programs, and how do they infect devices?

A Trojan is a malicious program that masquerades as genuine to deceive users into installing it. Active, it can steal data, spy on users, open backdoors, or download more malware.

Unlike worms, Trojans do not self-replicate; they rely on user action and attempt to remain undetected, compromising system security.

Key characteristics of Trojans:

• Social engineering: Trojans rely heavily on deception, such as fake emails or malicious software, to trick users into installing them.
• Manual installation requires the user to run a file or manually execute the malware.
• Hidden payloads: Trojans often contain additional malware, such as ransomware, keyloggers, or remote access tools (RATs).
• No self-replication: Trojans do not automatically spread; they have to be spread through phishing, downloads, or infected websites.

Common ways Trojans infect systems:

• Fake emails with spurious invoices, password reset messages, or shipping tracking links.
• Downloads from suspicious websites of “free” software, cracked programs, or pirated games.
• Fake software updates that prompt users to install patches for nothing.
• Malicious advertisements (malvertising) that automatically direct users to contaminated sites or auto-download malicious programs.

Real-world example: Emotet Trojan

Emotet initially emerged as a banking Trojan designed to steal users’ financial data. It later developed into a full-fledged malware delivery platform that could deliver ransomware, data stealers, and other Trojans on infected machines.

Despite multiple global takedown efforts (including a recent combined Europol operation in 2021), Emotet has returned several times with new approaches, showing the constant danger trojans pose.

Differences between Worms and Trojans

The differences include:

1. Spread method

• Worm: Automatically spreads across networks.
• Trojan: Requires user action, often via deception.

2. Host file required?

• Worm: No, it is a standalone program.
• Trojan: Yes, it is camouflaged within a seemingly valid file.

3. User interaction?

• Worm: Not necessary.
• Trojan: The necessary user needs to open or install the contaminated file.

4. Shared impact

• Worm: Can cause network overload, system slowdown, and resource drain.
• Trojan: Can lead to data theft, surveillance, backdoor access, and ransomware delivery.

5. Entry points

• Worm: Exploits unpatched software and system vulnerabilities.
• Trojan: Enters through phishing messages, malicious ads, or impostor downloads.

How worms and Trojans work together in cyberattacks

They include:

• Hybrid attacks: Emerging cyberattacks combine worms and trojans for maximum devastation.
• Trojan entry: Attackers use trojans, such as Emotet, to gain entry into systems via phishing emails or malware downloads, providing remote access to steal data or install malware.
• Worm spread: After the Trojan has infected a system, a worm is executed to spread and duplicate itself across the network, usually delivering ransomware to encrypt files.
• Amplified harm: Worms spread autonomously without user action, while Trojans enable focused data exfiltration and system compromise.
• Difficulty in detection: The hybrid model complicates detection because defenses might neutralize one threat (e.g., Trojan) but not another (e.g., worm).
• Emerging threats: These sophisticated multi-stage attacks reflect the need for stronger security controls to protect against evolving cyber threats.

Case study:

In 2017, the NotPetya malware combined worm-like propagation patterns with a ransomware payload, resulting in severe global damage. Initially spread through a Windows vulnerability (EternalBlue), it rapidly spread across networks autonomously, much like a worm.

Although it appeared to be ransomware demanding Bitcoin, it did nothing but wipe out data, not restore it.

The attack targeted major companies, including Maersk, Merck, and FedEx, resulting in over $10 billion in losses. Suspected to be state-sponsored, NotPetya highlighted the devastating capabilities of hybrid malware, with its self-replication and destructive properties, as justification for a more aggressive cybersecurity approach.

How to prevent malware infections from worms and Trojans

They include:

1. Update systems and software: Unpatched vulnerabilities are worms’ 1 point of entry.
Update:

• Operating systems
• Browsers
• Antivirus software
• Any third-party applications

Pro Tip: Enable automatic updates when possible.

2. Use advanced antivirus and anti-malware software: Sophisticated cybersecurity software contains behavior-based detection, which can identify advanced worms and trojans, even ones not yet supported in signature databases.
Look for products that offer:

• Real-time scanning
• Sandboxing suspicious files
• Practical analysis

3. Educate and train users: Cybersecurity awareness training is necessary since Trojans rely so heavily on human error.

• Train employees to recognize phishing emails.
• Alert them against installing software from anywhere other than a trusted source.
• Emphasize the importance of verifying unexpected attachments or links.

4. Implement network segmentation: Segmenting your network into sections minimizes the likelihood of worms spreading in the event of an attack. For example, if a department becomes infected, effective segmentation can prevent the malware from spreading to other departments.

5. Adopt a zero-trust security model: Zero-trust does not assume that any user or device is inherently trusted. Zero-trust employs continuous authentication and stringent access controls to reduce the opportunity for trojans and worms to move laterally.

Protecting yourself against malware threats

The world of the Internet is evolving, as are the cyber threats that lurk beneath. Worms and Trojans are two of the most malicious types of malware, employing distinct methods to infiltrate and damage your systems. Worms travel like lightning, exploiting weaknesses with no user intervention.

Trojans deceive users into allowing them to enter, only to unleash hidden payloads, such as ransomware or spyware, that can cause significant damage.

More concerning is the emerging pattern of hybrid attacks, where trojans and worms collaborate, leveraging stealth and numbers to evade defenses and maximize impact, as seen in attacks such as NotPetya and Emotet.

FAQs

Q. 1 Which is more dangerous: a Trojan or a worm?

Both are extremely dangerous, but in different ways. Worms are more likely to take down entire networks rapidly, while Trojans are more focused on stealth, stealing information, and acquiring persistent access.

Q. 2 Can a Trojan spread like a worm?

Yes, some trojans (especially newer ones) possess self-replication, allowing them to behave like worms during initial infection.

Q3. How can I know my system has a worm or Trojan infection?

Search for unusual activity, such as unusually slow behavior, unfamiliar software running, weird network behavior, unexpected pop-ups, or warnings from the antivirus.

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