Computer Malware: Types, Detection, and Prevention

Malware, short for malicious software, is any program designed to harm your computer, steal your data, or give attackers control of your system. From viruses that spread between files to ransomware that locks your data for payment, malware comes in many forms. Understanding what malware is, how it works, and how to protect against it is essential for anyone using a computer.

Types of Malware

Viruses

A virus is malware that attaches itself to legitimate files or programs. When you run the infected file, the virus activates and can spread to other files on your system. Traditional viruses often delete files, corrupt data, or slow down your computer. They spread when you share infected files with others, either through email attachments, USB drives, or file downloads.

Viruses are less common today than they once were, largely because modern operating systems have better security built in. However, they still exist, and the basic principle, malicious code hiding inside seemingly legitimate files, remains a threat.

Ransomware

Ransomware is one of the most damaging types of malware. It encrypts your files, making them completely inaccessible, and then demands payment (usually in cryptocurrency) to decrypt them. You might see a message saying your files have been locked and you need to pay within 48 hours or they'll be permanently deleted.

Ransomware often spreads through phishing emails with malicious attachments or links. Someone clicks on what looks like an invoice or shipping notification, and the ransomware installs itself. Within minutes, thousands of files can be encrypted.

The damage from ransomware can be devastating, especially if you don't have recent backups. Even if you pay the ransom, there's no guarantee you'll get your files back. Many victims pay and receive nothing. This is why prevention and regular backups are so critical.

Spyware

Spyware monitors your activity and collects information without your knowledge or consent. It might log your keystrokes to steal passwords, track which websites you visit, or access your webcam and microphone. The stolen information is sent to the attacker, who might use it for identity theft, financial fraud, or selling to other criminals.

Spyware often comes bundled with free software downloads. You install what you think is a legitimate program, and spyware comes along for the ride, hidden in the installation process. Some spyware is designed to be very stealthy, running in the background with minimal impact on system performance, making it hard to detect.

Trojans

A Trojan, named after the Trojan Horse from Greek mythology, is malware that disguises itself as legitimate software. You might download what appears to be a useful utility or game, but it's actually malware. Unlike viruses, Trojans don't spread themselves. They rely on tricking you into running them.

Once activated, a Trojan might create a backdoor in your system, allowing attackers remote access. They could then install additional malware, steal files, or use your computer as part of a botnet to attack other systems.

Adware

Adware displays unwanted advertisements, often in the form of pop-ups or browser redirects. While not always malicious in the same way as ransomware or spyware, aggressive adware can slow your system, track your browsing habits, and expose you to scam websites. Some adware is borderline legitimate, bundled with free software and disclosed in fine print, while other types are clearly malicious.

How You Get Infected

Understanding how malware spreads helps you avoid it. Most infections happen in predictable ways.

Email attachments are a common vector. You receive an email that looks like it's from a shipping company, your bank, or a coworker. It contains an attachment, maybe a PDF or Word document. When you open it, macros or embedded code execute and install malware. This is why you should never open unexpected attachments, even if they appear to come from someone you know.

Malicious downloads from websites are another major source. You're looking for free software, a movie, or a game. You download it from an unofficial site, and it comes packaged with malware. Sometimes the entire download is malware pretending to be the software you wanted.

Software vulnerabilities let malware install itself without you doing anything. You visit a compromised website, and malware exploits a vulnerability in your browser or a plugin like Flash or Java. This is called a "drive-by download." The malware installs automatically just from visiting the page. This is why keeping your software updated is so important, updates patch these vulnerabilities.

Removable media like USB drives can carry malware. You find a USB drive in a parking lot or borrow one from a friend. You plug it into your computer, and malware automatically runs. Some malware is specifically designed to spread via USB drives, infecting any computer the drive is plugged into and copying itself to the drive to reach more victims.

Signs Your Computer Might Be Infected

Malware doesn't always announce itself. Some infections are obvious, but others are designed to be stealthy. Watch for these warning signs.

Your computer runs much slower than usual, especially if the slowdown is sudden. Programs take longer to open, and simple tasks feel sluggish. While this can have innocent causes like too many programs running or a failing hard drive, it can also indicate malware using your system's resources.

You see unexpected pop-ups, especially when you're not using a web browser, or your browser's home page changes without your permission. Toolbars you didn't install appear in your browser. These are classic signs of adware or browser hijacker malware.

Programs crash frequently or behave strangely. Files disappear or become corrupted. You can't access certain features or settings that previously worked fine. Malware often interferes with normal system operations.

Your antivirus software is disabled and you can't turn it back on. Many types of malware disable security software to prevent detection and removal. If your antivirus suddenly stops working and won't restart, investigate immediately.

Your contacts receive emails or messages from you that you didn't send. This often means malware has accessed your email or messaging accounts and is using them to spread to your contacts.

You see new programs or browser extensions you didn't install. Check your list of installed programs periodically. Unfamiliar entries could be legitimate software you forgot about, or they could be malware.

Protection and Prevention

Use Antivirus Software

Antivirus software scans your files and running programs for known malware signatures and suspicious behavior. Windows comes with Windows Defender, which provides decent protection at no additional cost. macOS includes XProtect and Gatekeeper for similar protection.

For additional protection, consider third-party options like Bitdefender, Kaspersky, Norton, or Malwarebytes. These often include extra features like real-time web protection, ransomware shields, and behavior-based detection that can catch new malware that hasn't been cataloged yet.

Whatever antivirus you choose, keep it updated. New malware is created constantly, and antivirus software needs regular updates to recognize the latest threats. Most antivirus programs update themselves automatically, but verify this in your settings.

Keep Everything Updated

As covered in our software updates article, keeping your operating system, browsers, and applications current is one of the most effective malware defenses. Most malware exploits known vulnerabilities that have already been patched. If you're running outdated software, you're vulnerable to attacks that could have been prevented.

Be Cautious with Downloads and Email

Only download software from official sources. Get programs from the developer's website or official app stores. Avoid download sites that bundle multiple programs or offer "cracked" paid software for free. These are common malware distribution points.

Don't open email attachments from unknown senders. Even if the email seems legitimate, verify it before opening attachments. If your bank sends you an unexpected attachment, call them to confirm it's real. If a coworker sends something unusual, ask them about it before opening it.

Hover over links before clicking to see where they really go. A link might display "yourbank.com" as the text, but actually point to a completely different site. Check the actual URL before clicking.

Use Standard User Accounts

Don't use an administrator account for daily activities. Create a standard user account for regular use. Administrator accounts have permission to install software and change system settings, which means malware running under an administrator account has those same permissions. A standard account limits what malware can do even if it does infect your system.

Enable Your Firewall

Your firewall controls which programs can communicate over the network. Windows and macOS both include firewalls. Make sure yours is enabled. The firewall can block malware from communicating with command-and-control servers or spreading to other devices on your network.

Back Up Your Data

Regular backups won't prevent malware, but they protect you from the worst consequences, especially ransomware. If your files get encrypted by ransomware and you have a recent backup, you can restore your files without paying the ransom. Back up to an external drive or cloud service, and make sure your backup solution doesn't keep the backup drive connected continuously, or ransomware can encrypt your backups too.

If You Think You're Infected

If you suspect malware, disconnect from the internet immediately. This prevents the malware from communicating with its creators, spreading to other devices, or exfiltrating more data.

Run a full system scan with your antivirus software. If your antivirus is disabled or won't run, boot into Safe Mode and try again. Safe Mode loads only essential programs, which can prevent malware from running and interfering with the scan.

If your antivirus finds malware, follow its instructions to quarantine or remove it. After removal, run another scan to make sure everything is clean. Some malware installs multiple components, and removing one doesn't necessarily remove them all.

Change your passwords after cleaning your system, especially for important accounts like email and banking. If keylogging spyware was present, assume your passwords may have been compromised.

If you can't remove the malware or your system is badly compromised, you may need to reinstall your operating system from scratch. This is drastic, but sometimes necessary. Make sure to back up your personal files first, though be aware they might also be infected.

Malware is a serious threat, but it's also largely preventable. By keeping your software updated, using antivirus protection, being careful with downloads and emails, and maintaining regular backups, you can protect yourself from the vast majority of malware threats. Stay vigilant, stay updated, and don't let convenience override security.