Malware: Viruses, Worms, Trojans, Ransomware

Malware (malicious software) is any software intentionally designed to cause damage, gain unauthorized access, or disrupt systems. It is the primary tool of cybercriminals and nation-state actors.

A virus is malware that attaches itself to legitimate programs or files. It requires human action to spread (opening an infected file). When executed, it replicates and may corrupt files, steal data, or open backdoors.

A worm is self-replicating malware that spreads across networks without human interaction. It exploits vulnerabilities in network services. Famous examples: WannaCry (SMB), Conficker, Morris Worm.

A Trojan disguises itself as legitimate software. Unlike viruses, it doesn't self-replicate — it relies on the user to install it. Once installed, it may create backdoors, steal data, or download additional malware (droppers).

Ransomware encrypts victim files and demands payment for the decryption key. Modern ransomware operators also exfiltrate data before encrypting (double extortion) and threaten to publish it if ransom is not paid.

Examples: LockBit, REvil, Conti, WannaCry.

Spyware secretly monitors user activity — keystrokes, screenshots, browsing history — and transmits it to the attacker. Adware displays unwanted advertisements, often bundled with free software. Both can be combined with other malware types.

A rootkit hides its presence and that of other malware by modifying the operating system kernel or bootloader. Rootkits are extremely difficult to detect because they subvert the very tools used to find them. Detection often requires offline scanning.

A botnet is a network of infected machines (bots) controlled remotely by an attacker (botmaster). Bots are used for DDoS attacks, spam campaigns, credential stuffing, and cryptocurrency mining. Victims are often unaware their device is compromised.

Fileless malware lives entirely in memory (RAM) without writing files to disk, evading traditional file-based antivirus. It often uses legitimate system tools like PowerShell or WMI. Detection requires memory analysis and behavioral monitoring.

Common infection vectors:

• Phishing email attachments
• Malicious downloads from the web
• Drive-by download via compromised websites
• USB drops in parking lots
• Exploiting unpatched software vulnerabilities

Defense layers:

• EDR — behavioral detection and response
• Email filtering — sandbox attachments before delivery
• Patch management — close exploit opportunities
• Least privilege — limit malware damage scope
• Backups — offline copies for ransomware recovery

Which malware type spreads automatically across networks without requiring any user interaction?

Malware categories include viruses (require user action), worms (self-propagating), Trojans (disguised software), ransomware (encrypts files for ransom), rootkits (hide in OS), and fileless malware (memory-resident). Defense requires layered controls: EDR, email filtering, patching, least privilege, and backups.