What is Malware?
Malware—short for "malicious software"—is any software intentionally designed to cause damage to computers, servers, networks, or users. The term covers a broad range of threats including viruses, worms, trojans, ransomware, spyware, and more.
Malware can steal sensitive data, encrypt files for ransom, spy on user activity, conscript devices into botnets, or simply destroy systems. It's the primary tool of cybercriminals and nation-state hackers alike.
Types of Malware
Ransomware
Encrypts files and demands payment for decryption. The most financially damaging malware type. Learn more in our What is Ransomware? guide.
Trojans
Disguises itself as legitimate software to trick users into installing it. Once running, provides backdoor access, steals credentials, or downloads additional malware.
Infostealers
Designed to steal credentials, session cookies, cryptocurrency wallets, and other sensitive data. Major variants include RedLine, Raccoon, and Vidar.
Spyware
Secretly monitors user activity including keystrokes, browsing history, and screen content. Some spyware is used by governments for surveillance (e.g., Pegasus).
Worms
Self-replicating malware that spreads across networks without user interaction. Can cause massive damage quickly—NotPetya and WannaCry are famous examples.
Remote Access Trojans (RATs)
Provides attackers complete remote control over infected systems. Used for espionage, data theft, and deploying additional malware.
Botnets
Networks of infected devices controlled remotely. Used for DDoS attacks, spam campaigns, credential stuffing, and cryptocurrency mining.
Rootkits
Designed to hide deep in the operating system, evading detection. Provides persistent access and can survive system reboots and some security scans.
How Malware Spreads
- Phishing emails — Malicious attachments or links that download malware. See Phishing Email Examples
- Malicious websites — Drive-by downloads, fake software updates, and compromised legitimate sites
- Infected software — Pirated software, trojanized legitimate apps, malicious browser extensions
- Exploited vulnerabilities — Unpatched software exploited remotely, especially internet-facing systems
- USB drives — Infected removable media, sometimes deliberately dropped near targets
- Supply chain attacks — Malware inserted into legitimate software updates or development tools
Signs of Malware Infection
Warning Signs
- System running unusually slow or using high CPU/memory
- Unexpected pop-ups, ads, or browser redirects
- New programs or browser extensions you didn't install
- Antivirus disabled or unable to update
- Files missing, encrypted, or with strange extensions
- Unusual network activity or data transfer
- Password reset emails you didn't request
- Contacts receiving spam from your accounts
- System crashes or blue screens more frequently
How to Prevent Malware
Keep Software Updated
Enable automatic updates for your OS, browsers, and applications. Unpatched vulnerabilities are a primary entry point for malware.
Use Reputable Security Software
Install antivirus/EDR software and keep it updated. Built-in tools like Windows Defender are effective for most users.
Be Skeptical of Emails and Downloads
Don't open attachments or click links from unknown senders. Download software only from official sources.
Use Strong, Unique Passwords + MFA
Password managers make this practical. Enable multi-factor authentication everywhere it's offered.
Maintain Backups
Regular backups to offline or cloud storage ensure you can recover from ransomware and other destructive malware.
How to Remove Malware
- Disconnect from the network — Prevents spread and data exfiltration
- Boot into Safe Mode — Limits malware's ability to run and hide
- Run multiple antimalware scans — Use your primary AV plus a second-opinion scanner like Malwarebytes
- Remove suspicious programs — Check installed programs and browser extensions
- Check for persistence — Review startup programs, scheduled tasks, and services
- Change all passwords — Assume credentials are compromised, especially if infostealer suspected
- Consider rebuilding — For serious infections, a clean OS install is the surest fix
Frequently Asked Questions
Can Macs get malware?
Yes. While historically less targeted than Windows, macOS malware has grown significantly. Macs need security software and safe computing practices just like Windows PCs.
Can phones get malware?
Yes, especially Android devices which allow sideloading apps. iOS is more locked down but not immune, especially to sophisticated spyware. Stick to official app stores.
Is free antivirus good enough?
For most home users, yes. Windows Defender combined with safe computing habits provides solid protection. Paid options add features like VPNs and password managers.