1. Viruses
Viruses are self-replicating programs that attach themselves to legitimate files or software. When a user executes the infected file, the virus activates, inserting its own code into other programs, documents, or system boot sectors. Infection typically occurs through downloading infected email attachments, sharing infected USB drives, or visiting compromised websites. Modern viruses often mutate to evade signature-based detection, making them persistent threats that can corrupt data, slow system performance, and spread across networks. Polymorphic viruses change their code signatures each time they replicate, while multipartite viruses attack both files and boot sectors simultaneously.
2. Worms
Unlike viruses, worms are standalone malware that do not require a host file to spread. They exploit network vulnerabilities, operating system flaws, or weak passwords to replicate and propagate autonomously. Infection vectors include phishing emails with malicious links, unpatched software vulnerabilities, and peer-to-peer file sharing networks. Worms consume bandwidth, overload servers, and create backdoors for additional malware. The infamous 2003 Blaster worm exploited a Windows RPC vulnerability to infect millions of machines within minutes. Modern worms like Stuxnet demonstrate how worms can target industrial control systems, causing physical damage to infrastructure.
3. Trojans
Named after the Trojan Horse of Greek mythology, this malware disguises itself as legitimate, useful software—such as a game, utility tool, or security update—to trick users into downloading and executing it. Trojans do not self-replicate; instead, they grant attackers remote access, steal sensitive data (passwords, banking credentials), or install additional malicious programs. Common delivery methods include cracked software downloads, fake browser extensions, and malicious ads (malvertising). Banking Trojans like Zeus and SpyEye are designed specifically to intercept online banking sessions and hijack transactions. Remote Access Trojans (RATs) like DarkComet allow attackers to control the victim’s computer, record keystrokes, and activate webcams.
4. Ransomware
Ransomware encrypts files or locks the entire system, demanding a ransom—typically in cryptocurrency—for decryption keys. Infection occurs through phishing emails containing malicious attachments (PDFs, Word docs with macros), drive-by downloads from compromised websites, or exploiting Remote Desktop Protocol (RDP) vulnerabilities. Modern ransomware families like Ryuk, LockBit, and BlackCat use double extortion: they steal sensitive data before encryption and threaten to leak it if the ransom is not paid. Some variants target backups and shadow copies to prevent recovery. Healthcare systems, municipalities, and enterprises are frequent targets due to their reliance on data availability. The 2021 Colonial Pipeline attack caused fuel shortages across the US East Coast.
5. Spyware
Spyware operates covertly to monitor user activity and collect personal information without consent. It logs keystrokes, captures screenshots, tracks browsing habits, and harvests login credentials, financial data, and email contacts. Infection channels include bundled software installations (especially freeware and shareware), deceptive pop-up ads, and compromised browser extensions. Keyloggers are a subtype that records every keystroke, enabling attackers to capture passwords and credit card numbers. Spyware also includes stalkerware and spouseware, which are marketed as parental monitoring tools but are often used for non-consensual surveillance. Advanced spyware like Pegasus exploits zero-day vulnerabilities in mobile operating systems to infiltrate smartphones.
6. Adware
While less overtly destructive, adware bombards users with intrusive advertisements, often redirecting browsers to unwanted websites or injecting ads into search results and legitimate pages. Infection typically accompanies free software downloads, where the adware is bundled as an “optional offer.” Some adware modifies browser settings or installs toolbars that are difficult to remove. Although adware is primarily a nuisance, aggressive variants can degrade system performance, consume bandwidth, and collect browsing data for marketing purposes. More dangerously, certain adware strains can also install other malware or serve malicious ads that lead to exploit kits. Scareware is a related form that uses fake antivirus alerts to frighten users into purchasing unnecessary software.
7. Rootkits
Rootkits are among the most insidious malware types, designed to gain privileged, kernel-level access to an operating system while hiding their presence and other malicious activities. They modify system files, drivers, and registry entries to evade detection by antivirus software and endpoint protection tools. Infection vectors include phishing emails, software vulnerabilities, or physical access to the device. Once installed, rootkits can disable security features, intercept system calls, and provide remote control to attackers. Bootkits are a subtype that infects the Master Boot Record (MBR) or Unified Extensible Firmware Interface (UEFI), activating before the operating system loads. The Sony BMG rootkit scandal of 2005 demonstrated how even legitimate companies used rootkits for digital rights management, creating massive security vulnerabilities.
8. Fileless Malware
Fileless malware operates entirely in memory, leaving no traditional file on the disk that antivirus scans can detect. It exploits legitimate system tools like PowerShell, WMI (Windows Management Instrumentation), and .NET frameworks to execute malicious scripts directly in RAM. Infection begins with a small initial payload—often delivered via a phishing email or malicious webpage—that downloads and runs a fileless script. Because the malicious code resides only in memory, it persists through system processes and registers itself in startup scripts or scheduled tasks. Some ransomware, like Samsam, uses fileless techniques to encrypt files without leaving traces. Detection relies on behavior analysis and memory forensics rather than signature-based scanning.
9. Botnets and Bots
A botnet is a network of compromised computers (bots) controlled remotely by a central command-and-control (C2) server. Individual bots are infected through worms, Trojans, or drive-by downloads, often without the user’s knowledge. Once enslaved, the botnet can be used for Distributed Denial-of-Service (DDoS) attacks, spamming, cryptocurrency mining, credential stuffing, or as a proxy for launching further attacks. Botnets like Mirai exploit IoT devices (routers, cameras, DVRs) with default credentials, amassing hundreds of thousands of infected devices. Modern botnets use peer-to-peer (P2P) communication structures to resist takedown attempts. The GameOver Zeus botnet stole over $100 million from bank accounts before being disrupted by international law enforcement.
10. Rogue Security Software (Scareware)
Rogue security software, also known as scareware, masquerades as legitimate antivirus or system optimization tools. It presents fake scan results, claiming the computer is infected with numerous threats, then pressures the user into paying for a “full version” to remove the non-existent malware. Infection occurs through malvertising, fake browser alerts (“Your system is infected!”), or drive-by downloads from compromised sites. In some cases, the scareware itself contains actual malware or opens backdoors after installation. While often considered a form of Trojan, its psychological manipulation makes it distinct. Recent variants have evolved into ransomware hybrids, locking the screen until a payment is made.