Malware is software or firmware designed to harm a device, steal information, or perform actions without permission. Attackers commonly get it onto devices by tricking people into clicking links, opening attachments, or downloading fake software; they may also exploit flaws in outdated software. Once it runs, malware may steal data, monitor activity, disrupt a device, or encrypt files—but not every infection does all of these things.
What malware means
Malware is an umbrella term for code designed or used to compromise a device’s security. It can affect the confidentiality, integrity, or availability of data and systems. The name alone does not tell you exactly what happened: modern malicious programs can combine behaviors associated with several categories.
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- Viruses and worms can replicate and spread, though they do not necessarily spread in the same way.
- Trojan horses disguise malicious behavior as ordinary or useful software.
- Spyware monitors activity or gathers information.
- Some adware is malicious, but the label “adware” alone does not establish that a program is malware.
Phishing is different: it is a deception technique, not necessarily malware. A phishing message may try to steal a password, persuade someone to install malware, or do both.
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How attackers get malware onto devices
Deceptive links and attachments
An attacker may impersonate a company, colleague, or other trusted sender, then create urgency or curiosity to prompt a click, attachment opening, download, or login. A deceptive message can lead to malware, credential theft, or both. A familiar display name or logo does not prove that a message is genuine. Verify unexpected or urgent requests by contacting the supposed sender through a phone number or address you already know, rather than details in the message. FTC guidance on recognizing phishing.
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Fake or Trojanized software
A download advertised as useful software can conceal malware. Fraudulent security ads and alarming pop-up warnings may also try to persuade people to install a program, call a number, or grant remote access. Get software from sources you know and trust. Do not call a number shown in a warning pop-up or give a stranger remote access because of an unsolicited alert. The FTC’s malware guidance explains how to respond to suspected infections and avoid fake support tactics.
Compromised websites and software vulnerabilities
Some web attacks try to exploit a flaw in the visitor’s operating system, browser, or another application. NIST describes drive-by attacks in which a visitor reaches an infected website and the site attempts to exploit vulnerabilities on the host. Depending on the flaw and the device, malicious code or attacker tools may be installed without the familiar step of opening an attachment. NIST’s definition of a drive-by attack.
Updating operating systems, browsers, and applications helps close known vulnerabilities, but updates cannot guarantee protection against every attack. A CISA advisory illustrates how methods can be combined: in a reported Truebot campaign, actors used phishing attachments and redirect links and also exploited CVE-2022-31199 in Netwrix Auditor to gain initial access. That is an example of one campaign, not evidence that this route is common across malware incidents. CISA’s Truebot advisory.
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What attackers may do after malware runs
Malware can provide attackers with access to a compromised device or its data. CISA notes in its Malware Tip Card that “Most commonly, malware is designed to give attackers access to your infected computer.” What happens next depends on the malware and the attacker’s purpose. Possible behaviors include:
- Stealing usernames, passwords, financial details, or other sensitive information.
- Monitoring or controlling activity on the device.
- Sending unwanted messages or displaying unwanted ads.
- Disrupting system operation.
- Encrypting files and demanding payment for their return.
- Creating a foothold for additional malicious tools.
These are possible outcomes, not a checklist that applies to every infection. A phishing attempt that only captures a password may compromise an account without installing malware on the device.
Quick Recap
How to reduce the risk
- Keep your operating system, browser, applications, and security software updated. Where available, enable automatic updates.
- Be cautious with unexpected links, attachments, and requests to download files or log in.
- Download programs only from sources you know and trust; treat unexpected security warnings with skepticism.
- Verify urgent requests using a known, independent contact method.
- Use trusted security software and keep it updated. It can help detect or block some threats, but no single tool or habit guarantees that a device will stay uninfected.
What to do if you suspect an infection
- Stop using the affected device for sensitive accounts. Do not log in to banking, shopping, email, or other important accounts from it while you investigate.
- Update trusted security software and run a scan. Follow the software’s instructions for handling anything it detects.
- Secure accounts that may be exposed. From a device you trust, change potentially exposed passwords and enable two-factor authentication.
- Get help if the problem persists or the next step is unclear. Contact the device manufacturer or a support provider you already trust. Do not rely on a number in a pop-up warning or an unsolicited technical-support contact. FTC advice on removing malware.
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