Common Types of Malware: What They Are and How They Work

Malware is not a new problem, but it is a rapidly evolving one. The first computer viruses were written in the 1970s and spread via floppy disks. Today, malware attacks are sophisticated enough to disrupt entire nations, with global cybercrime costs to grow by 15% per year over the next five years.

This guide breaks down the common types of malware, what makes each one dangerous, and what organizations can do to protect against them.

What Is Malware?

Malware (short for malicious software) is any program or file intentionally designed to damage, disrupt, steal sensitive data from, or gain unauthorized access to computer systems, networks, or devices.

Malware is deliberately created to cause harm.

How malware has evolved

Malware has existed longer than the modern internet. Early viruses spread via floppy disks and were relatively simple in their construction and impact. Today’s malware is a different category of threat entirely:

• Attacks are increasingly automated and AI-optimized
• Malicious software can target individuals, enterprises, and critical infrastructure simultaneously
• Modern malware often combines multiple attack types in a single campaign
• Fileless malware and polymorphic malware are designed specifically to evade traditional antivirus software and endpoint detection tools

What malware targets

How Does Malware Spread?

Understanding how malware spreads is crucial for IT professionals to implement effective prevention strategies. Malware uses various infection vectors to compromise business networks and systems:

• Email attachments and phishing campaigns: The most common delivery method, targeting employees with malicious attachments or links
• Drive-by downloads: Malware automatically downloads when visiting compromised or malicious websites
• Removable media: USB drives, external hard drives, and other portable storage devices
• Software vulnerabilities: Exploiting unpatched security flaws in operating systems and applications
• Social engineering tactics: Manipulating employees to voluntarily install malware or provide access credentials
• Network propagation: Spreading laterally through connected systems once initial access is gained

Common Types of Malware

Attackers use a wide range of malicious programs, each designed to achieve a different objective. Some encrypt files and demand payment. Others steal sensitive information silently in the background. Being aware of what each type does is the first step toward defending against it.

1. Ransomware

Ransomware is malicious software designed to encrypt files on a victim’s computer or network, making critical data completely inaccessible until a ransom is paid in exchange for a decryption key.

It is one of the most financially damaging types of malware in circulation today, with targets ranging from hospitals and schools to large enterprises and government agencies.

How it works: Ransomware typically gains access through phishing emails or malicious attachments, then moves laterally across networks to maximize the scope of encryption before triggering.

Once files are encrypted, victims face a choice between paying the ransom or attempting recovery from backups.

Key risks:

• Permanent data loss if no clean backup exists
• Operational shutdown while systems are locked
• Financial extortion with no guarantee of data recovery

Suggested read: How To Recover From a Ransomware Attack

2. Computer viruses

A computer virus is a piece of malicious code that inserts itself into a legitimate application or file and executes when that application is run.

Like a biological virus, it replicates by attaching to other programs and files, spreading through infected email attachments, malicious downloads, and shared storage.

How it works: The virus lies dormant inside a legitimate file until the user runs it. Once executed, it replicates itself, corrupts or deletes files, and can serve as a delivery mechanism for additional malware. Unlike worms, viruses require user interaction to spread.

Key risks:

• File corruption and data loss
• Spreads rapidly through shared files and storage
• Can deliver additional malicious payloads

3. Worms

A worm is self-replicating malicious software that spreads across networks automatically, without requiring any user interaction. Worms target operating system vulnerabilities to install themselves and replicate from device to device, often consuming significant network resources in the process.

How it works: Once inside a network, a worm scans for other vulnerable devices and copies itself to them automatically. It is frequently used to deliver additional malware, launch DDoS attacks, or create backdoors for further exploitation.

Key risks:

• Spreads without any user action
• Can infect entire networks rapidly
• Often used to stage larger attacks

4. Trojan horse

A Trojan disguises itself as legitimate software or a desirable file to trick users into executing it. Unlike viruses and worms, Trojans do not self-replicate. They rely entirely on social engineering to convince users to install them willingly.

How it works: Once executed, a Trojan opens a backdoor that allows attackers to gain unauthorized access to the system, steal login credentials and password data, or download and install additional malware without the user’s knowledge.

Key risks:

• Difficult to detect because it appears legitimate
• Provides persistent backdoor access for attackers
• Frequently used to deliver other malware types

5. Spyware

Spyware is malicious software designed to collect sensitive information about users’ activities without their knowledge or consent. It operates silently in the background, continuously transmitting stolen data to external servers controlled by attackers.

How it works: Once installed, spyware monitors user behavior and captures login credentials, password data, banking information, and personal communications.

It is designed to remain undetected for as long as possible, allowing attackers to accumulate large amounts of sensitive data before the infection surfaces.

Key risks:

• Can operate undetected for extended periods
• Enables identity theft and financial fraud
• Often collects far more data than the victim realizes

Suggested read: How to Prevent Spyware From Infecting Your Devices

6. Adware

Adware tracks a user’s browsing activity to serve targeted advertisements.

While some adware operates within the terms of a legitimate software agreement, malicious adware is installed without the user’s knowledge and can redirect browser traffic to malicious sites and serve as a delivery vehicle for more harmful software.

How it works: Adware embeds itself in a browser or application and monitors browsing behavior. It generates revenue for attackers through aggressive ad delivery and can progressively degrade system performance the longer it remains installed.

Key risks:

• Can escalate to more serious malware delivery
• Degrades system and browser performance over time
• Often dismissed as a minor annoyance rather than a threat

7. Rootkits

A rootkit gives attackers remote control of a victim’s computer with full administrative privileges. Rootkits are specifically designed to hide their presence from the operating system and security software, making them among the most difficult types of malware to detect and remove.

How it works: Because the operating system recognizes rootkit processes as legitimate, traditional antivirus software often cannot detect them without specialized tools. Rootkits are used to maintain long-term hidden access, disable security software, and facilitate the installation of additional malware.

Key risks:

• Extremely difficult to detect and remove
• Provides attackers with full system control
• Can disable antivirus and endpoint detection tools

8. Keyloggers

A keylogger monitors and records everything a user types, capturing login credentials, password data, financial information, and private communications without the user’s knowledge.

The recorded data is transmitted to attackers for use in identity theft, financial fraud, and unauthorized account access.

How it works: Keyloggers can be delivered as standalone malware or bundled with Trojans and spyware as part of a broader credential theft campaign. They run silently in the background and leave few visible signs of infection.

Key risks:

• Silent operation makes detection difficult
• Captures credentials across every application and website
• Frequently used as part of larger attack campaigns

9. Fileless malware

Unlike traditional malware, fileless malware does not install any files on the victim’s system. Instead, it operates within processes native to the operating system itself, such as Windows Management Instrumentation or PowerShell, executing malicious code entirely in memory.

How it works: By operating within legitimate system processes, fileless malware evades antivirus software and endpoint detection tools that scan for malicious files on disk. It leaves minimal traces, making forensic investigation difficult after the fact.

Key risks:

• Traditional antivirus software often cannot detect it
• Leaves minimal forensic evidence
• Increasingly common in sophisticated malware attacks

10. Mobile malware

Mobile malware targets mobile devices and includes many of the same attack types found on desktop systems, including Trojans, ransomware, spyware, and adware, adapted specifically for Android and iOS environments.

How it works: Mobile malware most commonly spreads through malicious apps distributed outside official app stores, phishing emails, SMS phishing, and malicious advertising served through legitimate apps.

As personal devices increasingly access corporate systems and store sensitive information, mobile malware has become a serious threat for both individuals and organizations.

Key risks:

• Blurs the line between personal and corporate risk
• Often bypasses traditional endpoint detection tools
• Can compromise corporate access through personal devices

11. Bots and botnets

A bot is malicious software that performs automated tasks on command. When an attacker controls a large network of infected computers, this is called a botnet.

Individual infected devices within a botnet often show no obvious signs of compromise, making detection difficult.

How it works: Once a device is infected and added to a botnet, attackers can use it remotely to launch DDoS attacks, send phishing emails at massive scale, conduct credential stuffing attacks, or mine cryptocurrency without the owner’s knowledge.

Key risks:

• Infected devices show few visible signs of compromise
• Enables attacks of significant scale using victim infrastructure
• Individual devices bear the cost of the attacker’s activity

Suggested read: Avoid Crypto Scams & Protect Your Assets [Email Safety Tips]

12. Cryptojacking

Cryptojacking hijacks a device’s processing power to mine cryptocurrency without the device owner’s knowledge or consent. It is often dismissed as a performance issue rather than a security incident, which is precisely what makes it effective.

How it works: Cryptojacking is delivered through malicious websites or infected software.

Once active, it runs continuously in the background, using the victim’s system resources to generate cryptocurrency for attackers while the device owner experiences degraded performance, overheating, and higher energy consumption.

Key risks:

• Easily mistaken for a hardware or performance problem
• Can run undetected for long periods
• Consumes system resources and increases operating costs
  

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Lesser-known types of malware

Beyond the common types of malware, IT professionals should be aware of these emerging and specialized threats:

• Scareware: Fake security software that tricks users into purchasing unnecessary protection
• Infostealers: Specialized malware designed to harvest credentials, cookies, and sensitive data
• Logic bombs: Malicious code that activates when specific conditions are met
• Polymorphic malware: Malware that changes its code to evade detection
• Hybrid malware: Combination of multiple malware types for maximum impact

Signs of a Malware Infection

Identifying a malware infection can be difficult.

Malicious software often operates silently in the background, designed specifically to avoid detection for as long as possible. However, certain signs of unusual system behavior can indicate that a device or network has been compromised.

Not all of these signs definitively indicate a malware infection, but any combination of them warrants immediate investigation by security teams.

How to Prevent Malware Attacks

Most malware attacks exploit the same weaknesses, such as outdated software, careless clicks, and weak security practices. While no single solution can guarantee complete protection, combining good cyber hygiene with proactive defense measures greatly reduces your risk.

Technical controls

Keep software and operating systems updated

Outdated software is one of the most exploited attack surfaces for malware.

Regularly updating and patching software closes the security vulnerabilities that worms, Trojans, and other malware rely on to gain access. This includes operating systems, browsers, plugins, and all third-party applications.

Deploy antivirus and anti-malware software

Antivirus software and dedicated anti-malware software provide real-time protection against known threats and can detect unusual system behavior that may indicate a new or unknown infection.

Endpoint detection tools add an additional layer by monitoring for anomalous activity at the device level.

Implement email authentication

Since phishing emails and malicious attachments are among the most common malware delivery methods, securing your email domain is a direct line of defense against malware spread.

Implementing DMARC, SPF, and DKIM ensures that attackers cannot impersonate your domain to send phishing emails carrying malware to your customers and partners. Organizations that reach full DMARC enforcement at p=reject make it significantly harder for malicious actors to exploit their domain as a malware delivery vehicle.

Use network security controls

Network security tools including firewalls, intrusion detection systems, and web filtering can block connections to malicious websites, flag unusual network traffic, and prevent malware from communicating with external command-and-control servers.

Restrict administrative privileges

Limiting which users and processes have administrative access reduces the damage malware can do if it gains a foothold. Rootkits and other sophisticated malware attacks rely on elevated privileges to maintain persistent access and disable security software.

Organizational practices

• Conduct regular security audits and vulnerability assessments to identify and address weaknesses before attackers can exploit them
• Maintain regular, tested backups of critical data stored separately from primary systems so that ransomware attacks do not result in permanent data loss
• Develop and test an incident response plan so that security teams can move quickly to contain and remediate a malware infection when one occurs
• Enforce a policy of using only approved software from verified sources to reduce exposure to malicious downloads

Employee education

Educating employees on recognizing phishing attempts is one of the most impactful investments an organization can make in malware prevention.

Since human error remains the primary attack vector for malware delivery through phishing emails and malicious attachments, a workforce that can identify suspicious emails, links, and files before interacting with them dramatically reduces overall risk.

Training should cover:

• How to identify phishing emails and suspicious attachments
• Safe browsing practices and avoiding malicious websites
• How to report suspected security incidents
• The risks of using personal devices or removable media in work environments

Protect Your Email Domain Against Malware Delivery With PowerDMARC

Email is the most common vehicle for malware delivery. A domain without proper authentication is an open door for attackers to impersonate your brand and send phishing emails carrying malicious attachments to your customers, partners, and employees.

PowerDMARC helps organizations close that door. With DMARC, SPF, and DKIM in place, you prevent attackers from spoofing your domain, gain visibility into unauthorized senders, and build the email authentication foundation that every serious cybersecurity strategy requires.

Get started with PowerDMARC and make your domain one less attack surface for malware to exploit.

FAQs

1. Is all malware considered a virus?

No, while a virus is a type of malware that attaches to files, malware also includes worms, ransomware, Trojans, spyware, and other malicious software.

2. What is the strongest malware in the world?

“Strongest” depends on impact and reach. Malware like Stuxnet, WannaCry, and NotPetya caused widespread disruption and financial damage, making them some of the most powerful in history.

3. What is the hardest type of malware to detect?

Fileless malware and rootkits are among the hardest to detect because they operate in memory or hide system-level activities, often bypassing traditional antivirus software.

4. What type of malware is ILOVEYOU?

ILOVEYOU was a computer worm that spread via email in 2000. It infected over 10 million computers worldwide by tricking users into opening an email attachment titled “LOVE-LETTER-FOR-YOU.txt.vbs” and caused billions in damages by overwriting files and stealing passwords.

5.  What is 13 malware?

“13 malware” typically refers to scam pop-ups claiming your computer has “13 viruses” or similar fake warnings. These are scareware tactics designed to trick users into downloading fake antivirus software or calling fraudulent tech support numbers.

6. Which of the following is not a type of malware?

Legitimate software like operating systems, antivirus programs, web browsers, and productivity applications is not malware. However, malware can disguise itself as these legitimate programs, which is why downloading software only from trusted sources is crucial.

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Ahona Rudra

Domain & Email Security Expert at PowerDMARC

Ahona is the Marketing Manager at PowerDMARC, with 5+ years of experience in writing about cybersecurity topics, specializing in domain and email security. Ahona holds a post-graduation degree in Journalism and Communications, solidifying her career in the security sector since 2019.

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