Malware, short for malicious software, refers to programs or code designed to harm computers, networks, or users without consent. Unlike legitimate software that serves user interests, malware operates against them, often covertly, to achieve objectives such as data theft, system damage, unauthorized access, or financial gain for attackers. Malware includes various categories such as viruses, worms, trojans, ransomware, spyware, rootkits, and botnets, each with different behaviors and impacts. Understanding malware types, infection vectors, symptoms, and protection mechanisms helps users protect systems and respond to infections. This page provides a technical overview of malware categories, how malware spreads, detection methods, and protection strategies.
Malware Definition
Malware is software intentionally designed to cause harm, damage, or unauthorized access. Key characteristics include:
- Unauthorized Execution: Runs without proper user consent or knowledge
- Harmful Intent: Designed to damage systems, steal data, or provide unauthorized access
- Covert Operation: Often operates secretly to avoid detection and removal
- Varying Capabilities: May steal credentials, encrypt files, monitor activity, or provide backdoor access
Malware can affect any device with computing capabilities, including computers, mobile devices, servers, and Internet of Things (IoT) devices. Automated malware distribution means all users are potential targets regardless of size or importance, as malware operations are often economically motivated and target large numbers of systems simultaneously.
Types of Malware
Viruses
Computer viruses are malware that attach themselves to legitimate files and require user action to activate. They share characteristics with biological viruses:
- File Attachment: Viruses attach to legitimate files such as documents, programs, or scripts
- User Activation: Require user action to activate, such as opening an infected file or running a program
- Replication: Once active, copy themselves to other files and systems
- Payload: May corrupt files, slow systems, display messages, or destroy data when executed
Viruses spread when infected files are shared or executed. The ILOVEYOU virus from 2000 spread via email attachments, infected millions of computers, and caused significant damage by overwriting files.
Worms
Worms are self-replicating malware that spread automatically without user interaction:
- Autonomous Spreading: Do not need to attach to files or require user action to spread
- Network Propagation: Exploit security vulnerabilities to spread automatically across networks
- Resource Consumption: Can overwhelm networks with replication traffic
- Payload Delivery: Often carry additional malware such as ransomware or backdoors
The WannaCry ransomware from 2017 used worm capabilities to spread across networks, exploiting a Windows vulnerability to infect hundreds of thousands of computers worldwide within days.
Trojans
Trojans, named after the Greek myth, disguise themselves as legitimate software to deceive users into installing them:
- Deception: Appear to be games, utilities, cracked software, or helpful tools
- No Self-Replication: Unlike viruses and worms, do not self-replicate but rely on user installation
- Backdoor Creation: Often create secret access for attackers to control systems remotely
- Data Theft: May steal passwords, banking credentials, personal files, or other sensitive information
Common trojan types include banking trojans that steal financial credentials, Remote Access Trojans (RATs) that provide complete system control, and droppers that install other malware. Trojans are among the most common malware types because they rely on social engineering rather than technical vulnerabilities.
Ransomware
Ransomware encrypts files and demands payment for decryption keys. See what is ransomware for detailed information:
- File Encryption: Encrypts files, making them inaccessible without decryption keys
- Ransom Demands: Demands payment, usually in cryptocurrency, in exchange for decryption keys
- Time Pressure: Often includes countdown timers threatening permanent file deletion
- Double Extortion: Modern variants also steal data and threaten to publish it if ransoms are not paid
Ransomware can cause severe damage to individuals and organizations by making critical files inaccessible. Protection requires backups, security software, and user education.
Spyware
Spyware secretly monitors and collects information from infected systems. See what is spyware for details:
- Keyloggers: Record keystrokes, capturing passwords, messages, and other typed information
- Screen Capture: Take screenshots or record screen activity
- Webcam and Microphone Access: Monitor users through cameras and microphones
- Browser Monitoring: Track browsing history, searches, and online activity
- Stalkerware: Commercial spyware used for relationship surveillance and monitoring
Spyware can collect extensive personal information, compromising privacy and enabling identity theft or harassment.
Rootkits
Rootkits are stealth malware designed to hide themselves and other malware from detection:
- Deep System Access: Operate at the kernel or firmware level, below typical detection mechanisms
- Evasion: Designed to evade detection by security software through system-level manipulation
- Persistence: Survive reboots and are difficult to remove without reinstalling operating systems
- Privilege Escalation: Provide attackers with administrator-level system access
Rootkits can hide other malware, making them particularly dangerous as they enable long-term system compromise. Detection and removal typically require specialized tools and may require operating system reinstallation.
Botnets
Botnets are networks of infected computers controlled by attackers:
- Bot Infection: Devices become bots under attacker control
- DDoS Attacks: Used to overwhelm websites and services with traffic from multiple infected devices
- Spam Distribution: Send millions of spam emails from infected devices
- Cryptocurrency Mining: Mine cryptocurrency using infected device hardware resources
- Command and Control: Centralized control allows attackers to coordinate bot activities
The Mirai botnet infected hundreds of thousands of IoT devices in 2016 and was used to launch DDoS attacks that disrupted major websites and services.
Adware and Potentially Unwanted Programs (PUPs)
Adware displays unwanted advertisements, while PUPs are programs that may be unwanted but are not necessarily malicious:
- Adware: Displays intrusive advertisements, may redirect browsers, or track browsing behavior
- PUPs: Programs that users may not want, such as toolbars, browser extensions, or software that changes browser settings
- Bundled Software: Often installed alongside other software without clear user consent
How Malware Spreads
Malware uses various distribution methods called attack vectors:
Email-Based Distribution
Email is a common malware delivery method:
- Malicious Attachments: Infected documents, PDFs, ZIP files, or executables sent as email attachments
- Phishing Links: Links to malware download sites or drive-by download pages. See what is phishing for details
- HTML Smuggling: Malware assembled from code embedded in emails, bypassing attachment filters
- Social Engineering: Emails designed to trick users into opening attachments or clicking links
Email-based distribution relies on social engineering to convince users to execute malware. Users should be cautious with email attachments and links, especially from unknown senders or unexpected messages.
Malicious Downloads
Malware distributed through download sources:
- Pirated Software: Cracked games, applications, and media often contain malware
- Fake Software: Programs impersonating legitimate software such as fake browser updates or security software
- Supply Chain Attacks: Legitimate software infected during development or distribution
- Browser Extensions: Malicious or compromised browser extensions
- File Sharing Networks: Peer-to-peer networks and file sharing sites
Users should download software only from official sources and verify downloads when possible.
Drive-By Downloads
Malware that installs automatically when users visit websites:
- Exploit Kits: Automatically scan browsers and plugins for vulnerabilities and exploit them
- Malvertising: Infected advertisements on legitimate websites that deliver malware
- Compromised Websites: Legitimate websites that have been hacked to serve malware
Drive-by downloads can infect systems without user interaction, making software updates important for protection.
Removable Media
Malware distributed through physical media:
- USB Drives: Infected drives that auto-run malware when plugged into systems
- USB Drop Attacks: Attackers leave infected drives in public locations hoping users will plug them in
- External Hard Drives: Can spread infections between systems when connected
Network Propagation
Some malware spreads automatically across networks:
- Vulnerability Exploitation: Exploiting unpatched vulnerabilities in network services
- Weak Credentials: Brute-force attacks against weak passwords
- Shared Resources: Spreading through network shares or shared storage
Signs of Malware Infection
Common symptoms that may indicate malware infection:
- Encrypted Files with Ransom Messages: Files encrypted with ransom demands displayed (ransomware)
- Disabled Security Software: Antivirus or security software disabled and cannot be re-enabled (rootkit, trojan)
- Spam from Your Accounts: Friends and contacts receive spam messages from your accounts (account compromise, botnet)
- Unexpected Pop-ups: Pop-up advertisements appearing even when not browsing (adware, PUP)
- System Performance Issues: System very slow, high CPU usage when idle, or unusual resource consumption (cryptominer, botnet)
- Browser Changes: Browser homepage or search engine changed without user action (browser hijacker)
- Unusual Network Activity: Unexpected network connections or data transfers
- Missing or Modified Files: Files deleted, modified, or moved without user action
- New Programs or Icons: Unknown programs installed or desktop icons added
- Error Messages: Frequent error messages or system crashes
Not all symptoms indicate malware—some may be caused by legitimate software issues. However, multiple symptoms or severe performance issues warrant investigation and scanning.
Protection Against Malware
Security Software
Antivirus and anti-malware software provide protection:
- Real-Time Protection: Scan files as they are accessed, downloaded, or executed
- Regular Scans: Periodic full system scans to detect malware that may have bypassed real-time protection
- Definition Updates: Keep malware signatures and detection databases current (usually automatic)
- Behavioral Detection: Some software uses behavioral analysis to detect previously unknown malware
Users should use reputable antivirus software and keep it updated. Operating systems include built-in security software that provides basic protection, though additional layers may be beneficial.
Software Updates
Keeping software updated addresses security vulnerabilities:
- Operating Systems: Enable automatic updates for operating systems
- Browsers: Update browsers promptly when updates are available
- Applications: Keep all software updated, as outdated programs may have known vulnerabilities
- Firmware: Update router firmware, IoT device firmware, and other device firmware
Many malware infections exploit known vulnerabilities that have been patched in updates. Regular updates significantly reduce infection risk.
Safe Download Practices
Careful downloading habits reduce infection risk:
- Official Sources: Download software only from official websites or app stores
- Avoid Pirated Software: Pirated software often contains malware
- Verify Downloads: Check file hashes or digital signatures when available
- Research Software: Research software before installing, especially from unknown sources
- Bundled Software: Be cautious of bundled software and decline extra offers during installation
Email Security
Email security practices reduce email-based malware risk:
- Avoid Unexpected Attachments: Do not open attachments from unknown senders or unexpected messages, even from known contacts (accounts may be compromised)
- Verify Links: Hover over links to verify actual destinations before clicking
- Be Wary of Urgency: Pressure tactics and urgent language are common in malware distribution
- Disable Auto-Preview: Disabling email auto-preview can prevent some email-based attacks
Firewalls
Firewalls block unauthorized network connections:
- Enable operating system firewalls (Windows Firewall, macOS firewall)
- Use router firewalls as additional protection layers
- Consider hardware firewalls for advanced protection in business environments
Backups
Regular backups protect against data loss from malware, especially ransomware:
- 3-2-1 Rule: Maintain 3 copies of data, on 2 different media types, with 1 copy offsite
- Offline Backups: Disconnect backup drives when not in use to protect against ransomware encryption
- Cloud Backups: Automatic, offsite protection (though ensure cloud backups are not accessible from infected systems)
- Test Restores: Periodically verify that backups can be restored successfully
Malware Removal
If malware infection is suspected or detected, see malware removal guide for detailed steps. General response procedures include:
- Disconnect from Internet: Prevent data exfiltration and malware communication with command and control servers
- Avoid Account Logins: Do not log into accounts, as malware may be capturing credentials
- Boot into Safe Mode: Boot into safe mode to prevent malware from running during removal
- Run Malware Scan: Use reputable anti-malware software to scan and detect threats
- Remove Detected Threats: Follow scanner recommendations to remove detected malware
- Change Passwords: From a clean device, change passwords starting with email and financial accounts
- Monitor Accounts: Watch for signs of identity theft, unauthorized access, or fraud
- Consider Reinstallation: For serious infections, clean operating system installation may be safest
For persistent or severe infections, consider using multiple scanning tools as no single tool detects everything. Professional assistance may be needed for complex infections or data recovery.
Limitations of Protection
No protection method is perfect. Limitations include:
- Zero-Day Exploits: Previously unknown vulnerabilities that have no patches available
- Social Engineering: Malware that relies on user action rather than technical vulnerabilities
- Advanced Threats: Sophisticated malware designed to evade detection
- Supply Chain Attacks: Legitimate software infected during development or distribution
- Detection Gaps: No security software detects all malware immediately
Defense in depth—combining multiple protection layers—provides better security than relying on any single method.