Malware is one of the most common cybersecurity threats facing individuals and organizations. The scale of the threat continues to grow, with AV-TEST, an independent IT-security institute, reporting that it registers more than 450,000 new malicious programs every day.

Malware can take many forms, and each type can affect systems differently. Some malware steals sensitive information, while other types encrypt files, provide attackers with remote access, spy on users, or disrupt business operations.

Understanding the different types of malware can help individuals and organizations recognize potential threats and take appropriate steps to protect their systems and data.

 

What is malware?

Malware, short for malicious software, is software created with harmful intent. Cybercriminals use malware for purposes such as stealing credentials, spying on users, disrupting operations, taking control of devices, and demanding ransom.

Malware can infect computers, smartphones, servers, cloud environments, and other connected devices. It can be delivered through several routes, including phishing emails, malicious websites, compromised software, infected files, removable devices, and vulnerable systems.

 

Types of malware

Viruses

A computer virus is malware that attaches itself to a legitimate file, program, or document. According to CISA, viruses can be “activated when you open an attachment or click a link contained in an email message.” When the infected file is opened or executed, the virus can activate and attempt to spread to other files or systems.

Viruses may corrupt or delete files, disrupt system operations, or provide attackers with a way to perform additional malicious activities. Unlike some other forms of malware, a virus typically requires some form of user interaction to spread.

 

Worms

A worm is malware that can replicate itself and spread from one computer or device to another without requiring a user to open an infected file manually.

Worms often exploit vulnerabilities in operating systems, applications, or networks. Once inside an environment, they can spread rapidly across connected systems. Their ability to spread automatically makes worms particularly disruptive to organizations with large or interconnected networks.

 

Trojan horses

A Trojan horse, commonly called a Trojan, is malware that disguises itself as legitimate or useful software. According to Science Direct, Trojan horses evade security mechanisms “by exploiting legitimate authorizations of the system entity that invoked the program in the first place.”

For example, a malicious program could be presented as a software update, application, or document. Once the victim installs or opens it, the malware can perform malicious activities in the background.

Trojans are often used to establish a foothold on a device before attackers deploy other malware.

 

Ransomware

Ransomware is malware designed to prevent victims from accessing their files, systems, or data. The malware commonly encrypts files and demands payment in exchange for a decryption key. Modern ransomware attacks may also involve data theft, with attackers threatening to publish stolen information if the victim does not pay.

Ransomware can affect individuals, businesses, healthcare organizations, government agencies, and other institutions. The healthcare sector is particularly attractive to attackers because medical organizations hold large amounts of sensitive information and often rely on systems that need to remain available. According to Paubox’s 2025 State of Security Report, healthcare organizations experienced a 264% increase in ransomware attacks since 2018.

 

Spyware

Spyware secretly monitors a user's activity and collects information without their knowledge or consent. Depending on the type, spyware may collect browsing activity, login credentials, personal information, or other sensitive data.

The stolen information can then be sent to an attacker or used for identity theft, fraud, or further attacks.

 

Keyloggers

A keylogger is malware designed to record the keys a user presses on their keyboard. Attackers can use keyloggers to capture usernames, passwords, messages, financial information, and other sensitive information entered through a keyboard. As Microsoft states, “Keyloggers are a serious risk to personal and organizational security, silently recording keystrokes to steal sensitive information.”

Keyloggers can operate quietly in the background, making them difficult for users to notice.

 

Adware

Adware is software that displays unwanted advertisements on a device. Not all adware is malicious. Some legitimate applications use advertising as part of their business model. Malicious adware, however, can generate intrusive advertisements, redirect users to unwanted websites, or monitor browsing behavior. In some cases, adware can also serve as a pathway for other threats.

 

Rootkits

A rootkit is designed to hide malicious activity on a compromised system. Rootkits can provide attackers with privileged access while attempting to conceal files, processes, or other signs of compromise from users and security tools.

They can be particularly difficult to detect and remove because they are designed specifically to remain hidden.

 

Backdoors

A backdoor creates a way for an attacker to bypass normal security controls and access a compromised system. Some malware can install a backdoor after infecting a device. Once established, attackers may use the backdoor to remotely control the system, steal information, or install additional malware. A recent example involved the ValleyRAT backdoor being disguised within signed adware, allowing the malware to hide behind a legitimate-looking application and potentially evade security controls.

Backdoors can allow attackers to maintain access even after the original infection method is no longer available.

 

Bots and botnet malware

A bot is a compromised device that can be controlled remotely by an attacker. When large numbers of infected devices are connected and controlled together, they form a botnet. Cybercriminals can use botnets for activities such as distributed denial-of-service (DDoS) attacks, spam campaigns, credential attacks, and other malicious operations.

The owner of an infected device may not realize that their computer or smartphone is being remotely controlled.

A recent example is Sality, a 23-year-old peer-to-peer botnet that infected more than 15,000 machines worldwide and was used for credential theft, spam distribution, proxy services, network exploitation, and DDoS attacks. In September 2026, international law enforcement agencies, CrowdStrike, and the Shadowserver Foundation disrupted the botnet by isolating infected machines through a sinkhole operation, preventing attackers from communicating with the compromised devices.

In the news: FBI and DOJ disrupt China-linked hacking group targeting hospitals

 

Fileless malware

Fileless malware operates without relying primarily on traditional malicious files stored on a device. Instead, it can abuse legitimate tools and processes already available on the system. For example, attackers may use legitimate administrative or scripting tools to execute malicious commands. This approach can make detection more difficult because there may be fewer traditional malware files for security software to identify.

 

Cryptojacking malware

Cryptojacking involves secretly using another person's computer, server, or cloud resources to mine cryptocurrency. The malware consumes the victim's processing power and other system resources to perform cryptocurrency-mining operations for the attacker.

Signs can include unusually high CPU usage, slow system performance, increased electricity consumption, and unexpected resource usage in cloud environments.

 

Banking Trojans

Banking Trojans are designed specifically to steal financial information. They can monitor users while they access banking websites, steal login credentials or manipulate online banking sessions.

Some banking Trojans can also intercept authentication information, potentially allowing attackers to gain access to financial accounts.

 

Downloaders and droppers

Downloaders and droppers are malware components designed to deliver other malicious software. A downloader typically connects to an external server and retrieves additional malware after the initial infection. A dropper, on the other hand, may contain or unpack another malicious payload on the victim's device.

Attackers commonly use these techniques to separate the initial infection from the malware ultimately used during an attack.

 

Loaders

A malware loader is designed to establish the conditions needed for another malicious program to run. Loaders may attempt to evade security tools, establish persistence, or prepare a system for another malware payload. They are frequently used as part of multi-stage attacks in which different malware components perform different roles.

According to IBM, the malware loader, QuirkyLoader, has been “used to deliver additional payloads to infected systems.” This is an example of a multi-stage infection that begins with an email.

 

Remote access Trojans

According to the National Cybersecurity Alliance, a Remote Access Trojan (RAT) is “a type of malware that allows a hacker to control your device from anywhere in the world secretly.” It gives an attacker remote control over an infected device.

Depending on its capabilities, a RAT may allow attackers to access files, monitor activity, execute commands, install additional malware, or interact with the compromised system remotely.

In the news: Hackers impersonate Ukraine's cyber defense agency to deploy remote access trojan

 

Mobile malware

Mobile malware targets smartphones and tablets rather than traditional computers. It can include mobile banking Trojans, spyware, ransomware, adware, and information stealers. Mobile malware may attempt to steal messages, credentials, financial information, or other data stored on a device.

 

Browser hijackers

A browser hijacker changes a user's browser settings without appropriate authorization. It may change the default search engine or homepage, redirect searches, or send users to unwanted websites. Although browser hijackers are sometimes classified as potentially unwanted programs rather than traditional malware, malicious variants can create significant security and privacy risks.

 

Logic bombs

A logic bomb is malicious code designed to activate when a specific condition is met. For example, the malware could be programmed to activate on a particular date, after a specific event, or when a certain system condition occurs. Logic bombs can remain inactive for extended periods, making them difficult to identify before they trigger.

 

Wipers

A wiper is malware designed to destroy or permanently disrupt data rather than hold it for ransom. Wipers may delete files, damage system structures, or otherwise make computers unusable. Unlike ransomware, where attackers generally claim they will restore access after payment, the primary purpose of a wiper is destruction and disruption.

The New Jersey Cybersecurity & Communications Integrations Cell released a warning in June 2025 of the increased Wiper threat. According to the warning, “Threat actors use several techniques to destroy data, including overwriting files with other data (such as NULL or random bytes), encrypting files and destroying the decryption key, corrupting or overwriting the Master Boot Record (MBR), and corrupting the Master File Table (MFT).”

 

Malicious macros

Malicious macros use macros embedded in documents to execute harmful commands. Attackers may distribute documents through phishing emails and encourage victims to enable macros. Once enabled, the macros can execute malicious code or download additional malware. Modern security controls have reduced the effectiveness of some traditional macro-based attacks, but malicious documents remain a common threat.

 

Web shells

A web shell is a malicious script placed on a web server that allows an attacker to remotely interact with the compromised server. After gaining access, an attacker may use a web shell to execute commands, access files, modify websites, or install additional malware.

Web shells are particularly concerning because they can provide attackers with persistent access to internet-facing systems.

 

Bootkits

A bootkit is malware that targets the processes involved in starting a computer. By compromising the boot process, attackers may be able to execute malicious code before the operating system and some security protections are fully loaded. This can make bootkits particularly difficult to detect and remove.

 

Firmware malware

Firmware malware targets the firmware that controls hardware components. Since firmware operates at a lower level than the operating system, malware that compromises it can potentially remain persistent even if the operating system is reinstalled. Firmware attacks are less common than conventional malware infections but can present serious risks when they occur.

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How does malware spread?

According to IBM, malware can enter a device or network through several different attack vectors. Cybercriminals often rely on social engineering to trick users into downloading or opening malicious content, but malware can also exploit technical vulnerabilities or spread through compromised software and devices.

 

Phishing and social engineering

Phishing is one of the most common ways malware reaches victims. Attackers send convincing emails or messages designed to persuade recipients to open a malicious attachment, click a harmful link, or visit a website that delivers malware. IBM notes that phishing was a factor in 41% of malware infections, according to its X-Force Threat Intelligence Index.

 

Software and system vulnerabilities

Cybercriminals can exploit unpatched vulnerabilities in software, devices, and networks to introduce malware. Internet of Things (IoT) devices can be particularly vulnerable when they have weak or outdated security protections.

 

Removable media

USB drives and other removable devices can also spread malware. Attackers may deliberately leave infected USB drives in public places, hoping someone will connect one to their computer out of curiosity. Once connected, the malware can infect the device.

 

Fake software and downloads

Malware can be disguised as legitimate or useful software, including free applications, media files, antivirus programs, and performance-enhancing tools. Users who download and install these programs may unknowingly install malware at the same time.

 

Malvertising and drive-by downloads

Malvertising involves using malicious advertisements to distribute malware. Attackers may place harmful ads on legitimate advertising networks or create ads that appear to promote genuine software. Drive-by downloads can be even more deceptive, as malware may begin downloading when someone simply visits a compromised or malicious website, without deliberately downloading a file.

 

Compromised personal devices

Personal smartphones and laptops can become a route into an organization's network. An employee's device may become infected while outside the workplace and later introduce the malware when it connects to corporate systems.

 

Supply chain attacks

Malware can also spread through supply chain attacks, where attackers compromise a software provider, vendor, or service and use that trusted relationship to reach its customers. In one example cited by IBM, attackers exploited Kaseya's VSA platform to distribute ransomware to customers through what appeared to be a legitimate software update.

Learn more: What is a supply chain attack and how can it be prevented?

 

 

How Paubox can help

Paubox can help healthcare organizations reduce the risk of email being used to deliver malware and other threats that could ultimately compromise devices and contribute to botnet infections. Paubox Email Suite provides inbound protection against malware, ransomware, phishing, and spam, helping block malicious messages before they reach employees' inboxes.

Paubox also uses AI-powered threat detection to identify sophisticated phishing and impersonation attempts, including attacks that may bypass traditional email filters. This can help prevent attackers from gaining the initial access needed to compromise an employee's device.

For healthcare organizations, Paubox combines these email security controls with automatic encryption for outbound messages, helping protect sensitive information while reducing the likelihood that email becomes an entry point for cyberattacks.

See also: HIPAA Compliant Email: The Definitive Guide (2026 Update)

 

FAQS

How can I tell if a device has malware?

Potential warning signs include unexpected pop-ups, unusually slow performance, unknown applications, unexplained system changes, excessive resource usage, disabled security software, and suspicious network activity. These signs do not always mean malware is present, but they may warrant further investigation.

 

How can organizations protect against malware?

Organizations should use layered security measures, including endpoint protection, email security, regular software updates, strong access controls, multifactor authentication, employee security awareness training, network monitoring, and secure backups.

 

What should I do if I suspect a device is infected with malware?

Disconnecting the affected device from the network can help limit potential spread. Organizations should then follow their incident response procedures and have the device investigated by their IT or security team.