What Is Malware? The Main Types Explained

Malware — short for malicious software — is any program or code created to harm, exploit or gain unauthorized access to a computer, network or device. It is the umbrella term covering everything from viruses and worms to ransomware, spyware and rootkits. Almost every cyberattack involves malware at some stage, making it a central topic in threat intelligence.

Understanding the different categories matters because each type behaves differently, spreads differently and requires different defenses. Note that these categories overlap — a single piece of modern malware might be a trojan that drops ransomware and includes spyware features.

The main types of malware

Viruses

A virus attaches itself to a legitimate file or program and spreads when that host is executed and shared. Like a biological virus, it needs a host and a human action (running the infected file) to propagate. Viruses can corrupt data, steal information or deliver other payloads.

Worms

A worm is self-replicating malware that spreads across networks without needing a host file or human interaction, often by exploiting vulnerabilities. Because they propagate automatically, worms can infect huge numbers of systems extremely quickly — some of history's most damaging outbreaks were worms.

Trojans

A trojan (Trojan horse) disguises itself as legitimate, desirable software to trick users into installing it. Unlike viruses and worms, trojans don't self-replicate; they rely on deception. Once inside, a trojan can open a backdoor, steal data, or download additional malware. Many initial-access tools are trojans delivered via phishing.

Ransomware

Ransomware encrypts files or systems and demands payment for their release. It has become the most financially damaging category of malware — see our dedicated ransomware guide.

Spyware and infostealers

Spyware secretly monitors activity and collects information — keystrokes, screenshots, credentials. Infostealers are a booming subcategory designed to harvest passwords, cookies and crypto wallets, then sell them on criminal markets, often fueling later intrusions.

Rootkits and bootkits

A rootkit is designed to gain deep, privileged access and hide its presence — and the presence of other malware — from the operating system and security tools. Rootkits are notoriously hard to detect and remove because they subvert the very systems meant to find them.

Bots and botnets

A bot turns an infected device into a remotely controlled node; a network of them is a botnet. Botnets are rented out for spam, credential stuffing, and large-scale denial-of-service attacks.

Adware, scareware and fileless malware

Adware floods devices with unwanted ads; scareware frightens users into buying fake "fixes." Fileless malware is a stealthy modern technique that runs in memory using legitimate system tools ("living off the land"), leaving little on disk for traditional antivirus to find.

How malware spreads

• Phishing emails with malicious attachments or links.
• Drive-by downloads from compromised or malicious websites.
• Exploiting vulnerabilities in unpatched software.
• Malicious ads (malvertising) and fake software downloads.
• Removable media like infected USB drives.
• Supply-chain compromise, where trusted software is tampered with.

How to detect and defend against malware

Layered defenses cover prevention, detection and response:

• Endpoint protection and EDR. Modern endpoint detection and response catches malicious behavior, not just known signatures — crucial against fileless and novel malware.
• Patch management. Closing vulnerabilities removes a primary infection route.
• Email and web filtering. Block malicious attachments, links and sites.
• Least privilege and application control. Limit what code can run and what it can reach.
• User awareness. Most malware still relies on tricking a person.
• Backups. Tested, offline backups enable recovery from destructive malware.
• Threat intelligence. Track active malware families and their indicators and TTPs.

Signs of a malware infection

Malware increasingly tries to stay hidden, but infections often produce warning signs. Recognizing them early can mean the difference between a contained incident and a full compromise. Common symptoms include:

• Performance problems. A device that suddenly runs slowly, overheats or drains its battery may be running malicious processes in the background.
• Unexpected pop-ups and ads, especially outside the browser, are classic signs of adware or scareware.
• Programs crashing or behaving strangely, settings changing on their own, or new toolbars and applications you didn't install.
• Unusual network activity — spikes in outbound traffic or connections to unfamiliar servers can indicate command-and-control communication or data exfiltration.
• Disabled security tools. Malware frequently tries to turn off antivirus or block access to security websites.
• Locked files or ransom notes, the unmistakable sign of ransomware.
• Account anomalies — friends receiving spam from you, or logins from unexpected locations, suggesting credential-stealing malware.

The catch is that the most dangerous malware — the kind used by sophisticated actors — is specifically engineered to produce no obvious symptoms at all. Fileless malware running in memory and stealthy backdoors can sit quietly for months. That's precisely why organizations can't rely on users noticing something is wrong; they need EDR and monitoring that detect malicious behavior even when there are no visible signs. If you do suspect an infection, the safest steps are to isolate the device from the network, avoid logging into sensitive accounts from it, and engage your security team or a professional rather than simply deleting the obvious file — which may be only one component of a larger compromise. Early detection and proper response consistently limit the damage far more than any single cleanup tool.

Quick recap:

• Malware is the umbrella term for any malicious software designed to harm, exploit or gain unauthorized access to systems.
• The main types — viruses, worms, trojans, ransomware, spyware, rootkits, botnets and fileless malware — behave and spread differently and increasingly blend together.
• It spreads via phishing, drive-by downloads, exploited vulnerabilities, malvertising, removable media and supply-chain compromise.
• Defense relies on layered, behavior-based controls — EDR, patching, filtering, least privilege, backups and current intelligence — not signatures alone.

The bottom line

Malware is the broad category of malicious software, spanning viruses, worms, trojans, ransomware, spyware, rootkits, botnets and stealthy fileless techniques. Because families evolve constantly and increasingly blend behaviors, defense depends on layered, behavior-based controls plus current intelligence about what's circulating. Our live threat intelligence feed aggregates malware research and campaign reporting from dozens of authoritative sources, deduplicated and ranked by priority.