Malware is harmful software—or, in some technical definitions, firmware or hardware—deliberately placed in a system to cause harm or enable unauthorized activity. It can steal information, disrupt a device, show unwanted ads, or encrypt files for extortion. The names for malware describe different behaviors, and more than one label can apply to the same threat.
What is malware?
Malware is short for malicious software. NIST’s glossary includes definitions that vary by source: one covers hardware, firmware, or software intentionally included in a system for a harmful purpose; another describes software or firmware intended to perform unauthorized processes that adversely affect confidentiality, integrity, or availability. These are context-specific formulations, not one universal wording. See NIST’s malware glossary.
For desktop and laptop systems, NIST SP 800-83 Rev. 1 defines it this way: “Malware, also known as malicious code, refers to a program that is covertly inserted into another program with the intent to destroy data, run destructive or intrusive programs, or otherwise compromise the confidentiality, integrity, or availability of the victim’s data, applications, or operating system.” Murugiah Souppaya and Karen Scarfone published the guide in July 2013; it is a foundational reference, not a current evaluation of security products or every mobile platform. Read NIST SP 800-83 Rev. 1.
Malware can expose usernames, passwords, banking details, or other sensitive information; disrupt a device; display unwanted ads; or encrypt data and demand payment. The word describes the harmful code, not necessarily how it reached the device. For example, phishing is a deception method that may deliver malware through a link or attachment, but phishing itself is not necessarily malware.
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What are the different types of malware?
These categories describe behavior and can overlap. A malicious program might, for example, arrive disguised as a Trojan and then perform spyware-like monitoring.
- Virus: Attaches to files or system areas and spreads through infected material. CISA describes viruses as infecting files or system areas and making copies.
- Worm: Replicates itself; CISA’s description notes repeated replication within a computer.
- Trojan horse: Disguises itself as a legitimate file or program to persuade someone to install or run it.
- Spyware: Quietly gathers sensitive information or monitors activity and reports it to unauthorized parties.
- Adware: Displays unwanted ads or redirects searches. Adware is not always malicious, so only harmful forms belong under malware.
- Ransomware: Encrypts data and demands payment to restore access. NIST describes cases where a user is tricked into running a downloaded file, after which vulnerabilities may be exploited and the attack may spread through an organization.
- Rootkit: Tools associated with opening a back door or maintaining unauthorized access.
- Botnet: A collection of infected computers controlled remotely, potentially for data theft or attacks.
The virus, worm, Trojan, spyware, adware, rootkit, and botnet descriptions above follow CISA’s Malware Tip Card. NIST’s current glossary also lists viruses, worms, Trojan horses, spyware, and some adware among examples of malware. For ransomware context, see NIST’s ransomware guidance.
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How does malware get onto a device?
- Unexpected email or message: A phishing link or attachment can trick someone into downloading malware. Avoid opening unexpected links or attachments.
- Unsafe downloads: Unfamiliar sites offering free media or games and peer-to-peer file-sharing downloads can expose users to malware.
- Vulnerable software: In NIST’s ransomware example, an attacker can exploit software vulnerabilities after a user executes a malicious download; malware may then spread across an organization.
These routes are not proof that a device is infected. They explain why cautious downloads, current software, and care with unexpected messages matter. The FTC’s US-focused consumer guidance covers these risks and prevention steps at Malware: How To Protect Against, Detect, and Remove It.
How can I tell if my device has malware?
Possible warning signs include:
- A computer that becomes slow, freezes, or crashes.
- A changed browser home page, unwanted redirects, or unfamiliar toolbars and add-ons.
- An unusual amount of pop-up advertising or repeated operating-system error messages.
- Security or system tools, such as Task Manager or Activity Monitor, that have been disabled.
- Emails or social posts sent from your account that you did not write.
These symptoms can have other causes and do not confirm malware. Conversely, malware may remain unnoticed for a time, even when security software is installed. Treat the signs as a reason to check, not as a diagnosis.
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How do I protect my computer or phone from malware?
The FTC’s April 2025 consumer guidance is US-focused and recommends practical safeguards. Security tools can reduce risk, but no tool or step guarantees protection.
- Use security software and keep it updated. Turn on automatic updates if available and configure it to scan new files automatically. NIST defines antivirus tools as software and technology used to detect malicious code, prevent infection, and remove code that has infected a system; that definition does not establish the effectiveness or suitability of any specific product. NIST’s antivirus-tools glossary.
- Be cautious with unexpected links and attachments. If a message claims to come from a company, contact it using a route you already know is genuine rather than following the message’s link. Do not call support numbers shown in pop-up warnings.
- Avoid risky download sources. Do not download free media or games from unfamiliar sites or use peer-to-peer downloads when you cannot verify what you are getting.
- Scan removable storage. Scan USB thumb drives, external drives, and other external devices before using them.
These steps follow the FTC’s consumer advice on malware protection.
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- Physical Write-Protect Switch: Protect your files by locking the drive in "read-only" mode, preventing accidental overwriting and shielding it from viruses and malware.
- Massive Storage Capacity: With capacities up to 512GB, the Kanguru SS3 provides ample space for storing large files like music, photos, videos, and more.
- Durable and Compact Design: Built with a high-strength aluminium casing, this lightweight drive is both sturdy and portable, perfect for on-the-go file storage.
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What should I do if I think I downloaded malware?
- Stop logging in to sensitive accounts on the suspected device. This reduces the chance of exposing more credentials while you investigate.
- Update your security software and run a scan. Follow the security software’s instructions for handling anything it detects.
- Secure your accounts. Change passwords and enable two-factor authentication, especially if you may have entered credentials after the suspected download.
- Get help if needed. Contact the device manufacturer, trusted technical support, or someone knowledgeable. The FTC’s consumer advice is not a substitute for an organization’s incident-response process.
If files have been encrypted by ransomware, payment can be expensive and does not guarantee recovery, according to NIST’s ransomware guidance. That warning is not a recommendation about how an organization should handle an incident.
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