What is malware? Types, warning signs and how to remove it

Malware — short for malicious software — is harmful software installed on your device without your knowledge, as the FTC puts it. NIST's definition is broader: any hardware, firmware or software deliberately put into a system for a harmful purpose. Criminals use it to steal usernames, passwords, bank account numbers and Social Security numbers, to flood you with ads, or to lock your files and demand payment.

Is malware the same as a virus?

No — a virus is one kind of malware. A virus copies itself by attaching to other programs or files, where it hides until it's activated. "Computer virus" is often used loosely for any malware, and most security software deals with all the types below.

The main types of malware

• Viruses — attach themselves to other programs or files and run when those are opened.
• Worms — copy themselves and spread across a network on their own, often by exploiting security holes.
• Trojans — look like useful programs but hide a malicious function. Unlike viruses, they can't spread by themselves: you're tricked into installing them.
• Ransomware — locks or encrypts your files and demands a payment to restore them.
• Spyware — installed secretly to collect information about you without your knowledge.
• Keyloggers — record the keys you press to capture passwords and other details.
• Adware — floods you with ads. NIST counts some forms of it as malicious code.
• Rootkits — hidden changes that alter how the system works while concealing themselves.

Microsoft's list adds coin miners, which quietly use your computer to mine cryptocurrency, and tech support scams, which use fake warnings to get you to call a number.

How malware gets onto your devices

• Email attachments dressed up as a delivery receipt, a tax refund or an invoice.
• Programs bundled with other downloads — and pirated software or "keygen" tools, which often install malware at the same time.
• Bogus ads for software on social media and search engines, which the FTC warns about.
• Documents that ask you to enable macros.
• Infected USB drives and other removable storage.
• Hacked or malicious websites.

Warning signs

The FTC lists these signs that a computer may have malware:

• It slows down, freezes or crashes.
• Your browser's home page changes, or it keeps taking you to sites you didn't mean to visit.
• New toolbars or extensions appear that you didn't install — Google notes that unwanted extensions that keep coming back are a sign too.
• You see a flood of pop-up ads.
• Tools like Task Manager won't open.
• People get emails or posts from you that you didn't send.

Any one of these can have an ordinary cause. And security software doesn't catch everything: the FTC notes that malware can go undetected, at least for a while.

How to remove malware

1. Stop logging in to accounts that use passwords or sensitive information, such as banking and shopping.
2. Update your security software — or install it from a reputable vendor's official site — run a full scan and remove anything it flags.
3. On Windows, also run the offline scan in Windows Security (Virus & threat protection > Scan options). It runs before Windows loads, so persistent malware has a harder time hiding.
4. On a Mac, if macOS warns that a download is malware, move it to the Trash and empty the Trash.
5. Remove browser extensions you don't recognize, and reset the browser's settings if pop-ups and redirects continue.
6. Once the device is clean, change your passwords and turn on two-factor authentication.
7. If problems continue, contact the device maker's support or a technician you trust.

How to keep malware out

• Keep your operating system and apps updated, with automatic updates on.
• Download software only from the vendor's official website or the app store.
• Use a standard, non-administrator account for everyday use, as Microsoft recommends.
• Back up with the 3-2-1 rule: three copies, in at least two places, with at least one offline.
• Turn on multi-factor authentication for important accounts.
• Run one real-time antivirus, not two — Microsoft warns that several at once can make a computer slow or unstable.
• On Android, keep Google Play Protect on; it also checks apps installed from outside the Play Store.

Do you need extra antivirus?

Windows 10 and 11 include Microsoft Defender Antivirus, and Macs have their own built-in defenses — though Apple is clear that Macs can still get malware. A paid suite adds protection across all your devices from one account, warnings about scam and phishing sites, and extras such as a VPN or password manager. Our guide to whether Windows Defender is enough walks through when that's worth paying for.

Most malware is invited in — a download, an attachment, a cracked app. Be choosy about what you install, and keep one good layer of protection switched on.