What Is Malware? Definition, Types & How It Works

What is malware?

Malware is invasive software or code designed to infect, damage, or gain access to computers or mobile devices. There are many different types of malware, and each infects and disrupts devices differently, but all malware is designed to compromise security, privacy, or performance.

Some severe strains of malware target financial data and other sensitive information, and are used to commit extortion, fraud, and identity theft. And malware isn’t a threat only to Windows PCs — Macs and mobile devices are also vulnerable to malware attacks.

Malware definition

Malware is an umbrella term for any type of “malicious software” that’s designed to infiltrate your device without your knowledge, cause damage or disruption to your system, or steal data. Adware, worms, and rootkits all fall under the definition of malware.

Common types of malware

Malware is designed to disrupt, spy on, or exploit computer systems in different ways. Common types include ransomware, spyware, and Trojans — each with unique methods of attack and potential impact on users and organizations. Understanding the various types is key to identifying threats and protecting against them.

Viruses

A computer virus is malware that attaches itself to a file or program, self-replicates, then spreads to other systems or devices when a user shares the file. If the infected file is opened or downloaded, the virus is activated and carries out harmful actions, such as deleting data, damaging files, or stealing sensitive information.

Ransomware

Ransomware is the malicious software version of a kidnapper’s ransom note. It works by locking or denying access to a device or files until a ransom is paid. However, even if the ransom is paid, there’s no guarantee the hacker will return access or the files. Any individuals or organizations that store important information on their devices are at risk of a ransomware attack.

Spyware

Spyware collects information about a device or network and relays this data back to the attacker. Hackers typically use spyware to monitor a person’s internet activity and harvest sensitive data, including login credentials, credit card numbers, or financial information that can be used to commit identity theft.

Worms

Worms are designed with one goal in mind: proliferation. A worm infects a computer, then self-replicates, spreading to additional devices while remaining active on all infected machines. Some worms act as delivery agents to install additional malware, while others cause issues such as clogging networks, slowing down computers, and taking up storage space.

Adware

Adware is software designed to generate revenue by displaying unwanted advertisements on your device. It's often bundled with free software, games, or browser extensions and may also track your browsing activity or collect personal information to serve targeted ads. Beyond being intrusive, adware can compromise your privacy, slow down your device, and expose you to additional security risks.

Scareware

Scareware displays fake virus alerts to make users believe their device is infected. These pop-ups create a false sense of urgency to trick users into clicking dangerous links, entering payment details, visiting malicious sites, or installing fake antivirus software. This can result in malware being installed, sensitive information being stolen, financial loss, or unauthorized device access.

Trojans

Trojans are a type of malware used to conceal another type of malware. Trojan malware infiltrates a victim’s device by presenting itself as legitimate software. Once installed, the Trojan activates and can perform a range of malicious actions, such as stealing data, creating a backdoor for attackers, or downloading additional malware.

Trojan malware gets its name from the stories ancient Greek poets told of Athenian warriors hiding inside a giant wooden horse, then emerging after Trojans pulled it within the walls of their city.

Cryptojackers

Cryptojackers are a type of malware that secretly uses a victim’s device to mine cryptocurrency. By hijacking the processing power of laptops, phones, and other devices, cybercriminals can generate cryptocurrency without using their own hardware and resources.

Cryptojacking malware is commonly spread through phishing emails, malicious downloads, software vulnerabilities, or infected websites. Because cryptocurrency mining requires significant processing power, infected devices often become slower, consume more electricity, overheat, and experience rapid battery drain.

Keyloggers

A keylogger, short for “keystroke logger,” is a type of malware that records everything typed on a device. It can capture passwords, messages, and other sensitive data, allowing attackers to access accounts, steal money, scam users and their contacts, sell personal data on the dark web, or commit identity theft.

Fileless malware

Fileless malware infects a device without installing traditional files. Instead, it runs primarily in a computer’s random access memory (RAM) and abuses trusted tools or software already on the system. For example, it might use PowerShell to exfiltrate data or monitor user activity.

Because it leaves little to no trace on a device, it can slip past traditional signature-based antiviruses. Modern anti-malware solutions that use behavior-based detection are better equipped to spot subtle changes that could indicate fileless malware.

Modern malware is both varied and sophisticated, which means protecting yourself against every threat manually just isn't realistic. But thankfully, modern free antivirus software like Avast One uses advanced technologies such as behavior-based detection to identify even the very latest malware, and help detect, block, and remove threats before they put your devices and data at risk.

How does malware work?

Although the various types of malware behave differently, they generally follow a similar pattern: a device becomes infected when malicious code is delivered through a deceptive download, a malicious link, a compromised website, an infected USB stick, or the exploitation of a software vulnerability.

Once installed, malware can steal sensitive data, monitor your activity, damage or encrypt your files, take control of your device, or spread to other systems. Its specific behavior depends on the type of malware and the attacker’s objective.

How does malware get on your device?

Malware infections often occur after you unknowingly trigger a malicious download or file. Attackers commonly use social engineering tactics to pressure you into acting without thinking, such as clicking a malicious link, visiting a compromised website, or downloading infected software. In some cases, attackers can even exploit security vulnerabilities to install malware without any interaction from you.

By downloading malicious software

Malware can infect your device when you download software that appears safe or legitimate. This often happens when users install free programs, cracked software, fake updates, or apps from untrusted websites. Once downloaded, the software secretly installs harmful code alongside the intended program.

By clicking a malicious link

Malware can infect your device when you click a malicious link in an email, text, social media post, ad, or website. The links and messaging often appear legitimate, using familiar branding to trick you into trusting them. Clicking them may trigger an automatic malware download, redirect you to infected websites, or prompt you to install harmful software.

By visiting an infected website

Some infected or compromised sites contain malicious code that automatically downloads malware onto your device, without you clicking anything. Others may display deceptive pop-ups, like fake virus alerts, or download prompts that trick you into installing harmful software.

By using peer-to-peer file-sharing

Peer-to-peer (P2P) file-sharing networks allow users to download files directly from others. Cybercriminals may use P2P sites to disguise malware as popular movies, music, games, or software. When you download and open these files, they can install malware on your device.

By connecting an infected USB stick

Malware can spread through infected USB devices. Connecting an unfamiliar USB device to your computer may allow malware to execute automatically, or you might open an infected file. For this reason, you should never plug unknown USB drives into your devices.

Through exploit kits

Exploit kits are tools used by cybercriminals to scan devices for security weaknesses to deliver malware. When you visit a compromised or malicious website, an exploit kit might check for outdated software, browsers, or plugins. If a vulnerability is found, it silently installs malware automatically.

Through scam messages

Malware can infect your computer through phishing emails or your phone through smishing text messages. These messages often contain urgent alerts or tempting offers with links or attachments. When you click the link or download the file, malware can be installed.

Through botnets

A botnet is a network of infected computers, or “bots,” that follow commands from a controller. They’re sometimes used to send spam, which may include malware or dangerous links. Botnets can also be the first stage of a larger malware attack, since the attacker can infect all the computers on their network.

What does malware do?

Once malware infects a device, it tends to operate in the background, without the user’s knowledge. Here’s what it can do:

• Delete or damage data. Some malware erases files or corrupts data, causing damage and disruption.

• Steal sensitive information. A hacker may use malware to steal passwords, financial information, or personal data to commit financial fraud or identity theft.

• Consume system resources. Some malware causes significant performance issues like overheating, slow computer performance, freezes, or crashes.

• Create backdoor access. Some malware can create hidden entry points that allow hackers to remotely control devices, install additional malware, or monitor user activity.

• Display unwanted ads. Adware floods devices with pop-ups and may track browsing activity to display targeted ads.

• Compromise system security. Advanced malware can disable security software, change system settings, or block updates, making it more difficult to remove.

• Spread to other devices. Some malware can replicate itself and spread across networks, emails, removable media, or connected devices, increasing the scale of an attack.

• Hold files hostage. Ransomware encrypts files or locks users out of their devices and demands payment in exchange for restoring access.

Why is malware dangerous?

Malware is dangerous because it can compromise your privacy, finances, data, and device security. Depending on the type of malware, the consequences can range from intrusive ads and slower performance to stolen passwords, financial fraud, identity theft, or complete loss of access to your files.

The rise of Malware-as-a-Service (MaaS) has also lowered the barrier to cybercrime. Instead of developing malware themselves, less-skilled attackers can rent ready-made tools that may include malware builders, control panels, and even technical support. Subscriptions can cost as little as $150 and $1,000 per month, making sophisticated attacks accessible to a much wider pool of criminals.

Signs of malware

Signs of a malware infection include pop-ups, low storage space, changes to settings, unexpected device behavior, and performance slowdowns. While these symptoms don’t always mean your device is infected, they’re common warning signs and shouldn’t be ignored.

Here’s a run-down of the warning signs and why they can indicate malware:

• Your device runs slower than usual: Malware can consume processing power and memory, leaving fewer resources for legitimate apps.

• You're running out of storage: Some malware downloads additional files or replicates itself, gradually consuming disk space.

• You see pop-ups or unfamiliar programs: Persistent ads, unknown apps, or unexpected browser extensions can indicate adware or other malware.

• Your device crashes or freezes: Malware can interfere with normal system processes, causing apps or your operating system to become unstable.

• You notice unusual network activity: Unexpected spikes in data usage may indicate malware communicating with attackers or sending information from your device.

• Settings change without your permission: Malware can modify your homepage, search engine, security settings, or other system configurations.

If you notice one or more of these signs, use trusted antivirus software to scan your device and detect and remove any malicious software.

If you're seeing lots of pop-ups or your device has suddenly slowed down, it could be a malware infection.

History of malware attacks

Malware emerged in the 1980s, but it wasn’t until the Windows PC boom of the 1990s that malware threats really exploded onto the scene.

Here’s a look at some of the most important events and developments in the history of malware:

• 1982:The Elk Cloner virus, deployed against Apple II systems via floppy disk, is arguably the first “modern” malware attack.

• 1990: The term “malware” to describe malicious software is coined by cybersecurity analyst Yisrael Radai.

• 1995: Viruses targeting the Windows 95 operating system and applications became widespread, often hiding within document templates.

• 2002: Self-replicating malware known as “worms,” such as JS/Exploit-Messenger, spread across instant messaging networks.

• 2005: Adware attacks increasingly bombard users’ screens with annoying pop-ups and windows, some of which are also malvertising attacks that contain malicious code themselves.

• 2008: Hackers begin exploiting the growth of social media, using infected links and phishing attacks to spread all sorts of malware.

• 2010: The Stuxnet worm demonstrates the devastating impact malware can have on physical infrastructure.

• 2013: The CryptoLocker attack heralds the emergence of ransomware as a method of extortion, and Trojan delivery as a difficult-to-detect attack vector.

• 2016: Malware such as Mirai begins targeting smart home devices to create giant networks of “zombie” devices called botnets.

• 2019: Ransomware reemerges as one of the most widespread and potent malware threats, with several high-profile, devastating ransomware attacks targeting governments, companies, and other institutions.

• 2022: Hackers use malware exploits to compromise customer information in some of the biggest ever corporate data breaches.

• 2025: A Gen Threat Report found that the biggest malware trend wasn't the emergence of new malware types, but how attacks increasingly blend into everyday digital routines, with automation and AI helping cybercriminals target more people with less effort.

• 2026: SentinelOne malware stats for 2026 show that AI-generated phishing lures have increased click-through rates by up to 54%.

Modern malware examples

• DragonForce ransomware: Active since 2023, DragonForce is a ransomware‑as‑a‑service (RaaS) group. In 2025, it rebranded itself as a ransomware “cartel” to further grow its presence in the ransomware space.

• DroidLock: This Android ransomware locks users out of their phones, then displays a message demanding a ransom be paid within 24 hours, or else the attacker will delete all files.

• Lumma infostealer: Designed to steal banking info, passwords, and cryptocurrency wallet details, this was one of the most popular infostealer families before authorities disrupted its infrastructure in 2025. While its original infrastructure was weakened, it still remains an active threat.

• RomCom: First appearing in 2022 as a remote access Trojan, RomCom has evolved into a flexible malware platform, making it easier for attackers to tailor new campaigns without rewriting large portions of code.

Which devices can get malware infections?

No device is immune to malware — desktops, laptops, mobiles, and tablets are all susceptible. Along with securing your home network with a firewall shield, make sure each of your devices is protected with reliable anti-malware software, like Avast One.

Windows computers

The internet is awash with Windows PC malware such as WannaCry ransomware, and new zero-day vulnerabilities are being discovered and exploited all the time. Microsoft identified GigaWiper in October 2025, which encrypts or wipes systems. Its research determined that GigaWiper was created using three separate malware families.

These novel threats mean you should secure your computer with the best free antivirus software to help block and remove malware from your PC in real time.

Macs

Contrary to popular belief, Macs can get malware. It can spread through fake apps, compromised downloads, malicious extensions, and deceptive websites — just like malware on other platforms.

In late 2025, it was discovered that Macs were being targeted by malware impersonating more than 100 password managers. These were spread through fake GitHub repositories, highlighting the importance of downloading software from official stores like the Mac App Store.

Android phones

Android mobile devices can be infected with malware too, such as the DroidLock ransomware example above. Many types of mobile-specific malware are spread via SMS or fake app downloads. Other attack vectors include infected pop-ups, phishing emails, and drive-by attacks on unsafe websites.

iPhones and iPads

While iOS malware is rare, iPhones and iPads are still vulnerable to malware threats, like Pegasus spyware, and other security threats. For example, using unsecured public Wi-Fi can make it easier for hackers to access your device, although encrypting your connection and hiding your IP address with a VPN can help prevent this.

How to protect against malware attacks?

The best way to protect against malware is to follow cybersecurity best practices and install comprehensive anti-malware software. It can help detect, block, and remove malware.

1. Be careful with links and downloads

Malicious links, attachments, and downloads are among the most common ways malware reaches your device. A few simple checks before you click or install anything can significantly reduce your risk:

• Avoid suspicious links, attachments, and downloads: They may lead to malicious websites or install malware on your device.

• Skip cracked, pirated, or unofficial software: These downloads can be modified or bundled with hidden malware.

• Use official app stores: Download mobile apps from the Apple App Store or Google Play Store, which screen apps for security threats.

• Check ratings and reviews: Before installing unfamiliar software, look for credible reviews and warning signs such as consistently poor ratings or reports of suspicious behavior.

2. Practice safer browsing habits

The websites you visit and the content you interact with can expose you to malware, phishing, and other threats. Building safer browsing habits can help you recognize and avoid these risks:

• Avoid suspicious pop-ups and banner ads: They can redirect you to malicious websites or trigger unwanted downloads.

• Check URLs carefully: Before clicking a link or entering personal information, make sure the address is legitimate. Fake or spoofed websites can steal data or distribute malware.

• Be cautious with P2P file sharing: Files shared directly between users often aren't verified, increasing the risk of downloading malicious or compromised content.

3. Protect your device with updates and an antivirus

Keeping your software current and using reliable security tools creates a stronger defense against malware. These measures can help close vulnerabilities and stop threats before they compromise your device:

• Keep your software and operating system updated: Install updates promptly to patch known security vulnerabilities that malware can exploit.

• Use trusted anti-malware software: Protect all your devices with reputable security software. Tools like Avast One can help detect, block, and remove malware while also protecting against online scams.

4. Protect your accounts and data

Malware can put both your online accounts and valuable files at risk. Strengthening account security and maintaining reliable backups can limit the damage if your device or credentials are compromised:

• Use strong, unique passwords and enable MFA: Strong passwords make accounts harder to crack, while multi-factor authentication adds protection even if your password is exposed.

• Back up important files regularly: Keep secure backups of valuable data so you can recover it if malware deletes, corrupts, or encrypts your files.

How do I remove malware?

In most cases, malware can be removed and your device restored to normal. The simplest approach is to use anti-malware software, although you can also remove threats manually by uninstalling suspicious programs, deleting infected files, or, as a last resort, performing a factory reset.

To remove malware automatically, use a dedicated malware removal tool to scan your device, identify threats, and remove them. This is generally the easiest and safest approach, particularly when you don't know where the malware is hiding.

To remove malware manually, the exact process depends on your device. Typically, you'll need to disconnect from the internet, restart in Safe Mode where supported, remove suspicious apps and files, and restart normally. Follow the appropriate guide for detailed instructions:

• Remove malware from a PC

• Remove malware from a Mac

• Remove malware from Android devices

• Remove malware from iOS devices

Improve your malware protection

The strongest defense against malware is a robust security app from a provider you can trust. Avast One provides comprehensive online security to help keep all your devices safe. Download it now to benefit from advanced malware protection and stay safer from malicious websites and downloads, for free.