Malware Types Explained: Viruses, Trojans, Ransomware, and More

Classification: PUBLIC
Category: Threat Intelligence
Reading Time: 4 min

What Is Malware?

Malware (malicious software) is any program designed to damage, disrupt, or gain unauthorized access to computer systems. The malware landscape has evolved dramatically from simple boot sector viruses in the 1980s to today's sophisticated, multi-stage attacks used by nation-state actors and organized cybercrime groups.

Major Malware Categories

Viruses

A virus is a self-replicating program that attaches itself to legitimate files or programs. It requires human action to spread — opening an infected file, running a compromised program, or booting from an infected drive. Unlike worms, viruses cannot propagate on their own.

Worms

Worms are self-propagating malware that spread across networks without any user interaction. They exploit vulnerabilities in network services, operating systems, or applications to infect new hosts automatically. The WannaCry attack in 2017 used the EternalBlue exploit to spread across networks as a worm, infecting over 230,000 computers in 150 countries.

Trojans

Named after the legendary Trojan Horse, trojans disguise themselves as legitimate software to trick users into installing them. Once executed, they can create backdoors, steal data, or download additional malware. Common delivery methods include fake software updates, pirated applications, and email attachments.

Ransomware

Ransomware encrypts the victim's files and demands payment (usually in cryptocurrency) for the decryption key. Modern ransomware operations use double extortion — they steal data before encrypting it and threaten to publish it if the ransom isn't paid.

Notable examples include LockBit, BlackCat (ALPHV), and Cl0p, which have collectively caused billions of dollars in damages.

Spyware

Spyware secretly monitors user activity, collecting keystrokes, screenshots, browsing history, and credentials. Advanced spyware like Pegasus can compromise mobile devices with zero-click exploits, requiring no user interaction at all.

Rootkits

Rootkits modify the operating system to hide their presence and maintain persistent access. They operate at the kernel level, making them extremely difficult to detect and remove. Some rootkits can even survive operating system reinstallation by infecting the UEFI/BIOS firmware.

Detection and Prevention

Keep Software Updated — Most malware exploits known vulnerabilities that patches have already addressed
Use Endpoint Detection and Response (EDR) — Modern EDR solutions use behavioral analysis to detect malware even without signature matches
Email Filtering — Block malicious attachments and URLs at the email gateway
Network Segmentation — Limit lateral movement by separating critical systems from general-purpose networks
File Integrity Monitoring — Detect unauthorized changes to system files and configurations
Hash Verification — Always verify file integrity by comparing hashes before executing downloaded files

Use Aegis Core's Hash Generator to verify file integrity by computing and comparing cryptographic hashes of downloaded files.