9 examples of malware and our recommendations for protecting your system

By Ainhoa Carpio-Talleux

Published: June 2, 2025

Want to protect your business against malware? Discover our complete guide to all you need to know about these harmful and dangerous computer programs. Examples of malware, how it works, what it can do to your system, recommendations... Find out all you need to know on the subject! 🛡️

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What is malware?

Definition of malware

As its name suggests, malware is a type of software specially developed to harm a system. It can take many different forms, but always pursues the same goal: to do damage. This can mean :

discreetly spying on your activity,
enabling a computer attack,
recovering your sensitive data,
corrupting your files,
replicate, etc.

In all cases, your computer system will pay the price.

🤔 What's the difference between malware and standard software?

The distinction between malware and standard software is very simple, they are the exact opposite

Firstly, the software provides a useful service whereas the malware only does harm.
Secondly, software is installed with the user's consent, while malware is installed without the user's knowledge.
Lastly, the software comes from a reliable publisher, while the malware is distributed via infected or dubious sources.

The purpose of malware

The main purpose of malware is to harm a system, but this harm can take many forms, depending on the cybercriminal's objectives:

It damages files or disrupts system operation.
Stealing sensitive information and banking data.
Spying on a user's online activities without their knowledge.
Takes full control of the device.
Generates profit via ransomware or unsolicited advertisements.

The 9 main types of malware

For every malicious action, there is a dedicated malware. Here's a list of the 9 main software categories and how they work, to help you detect them.

1- Computer virus

Computer viruses are the most common type of malware. It acts like a classic virus, but in the digital world. Specifically, it attaches itself to a host file or program. If you download this element onto your computer, the virus remains inactive until you execute the element in question. Hence the importance of analyzing each program before installation with a high-performance anti-malware program. Once executed, the virus carries out its intended actions (stealing passwords, sending spam to your contact list, etc.). A virus can also spread across an entire network.

Example of how a computer virus works:

2- Trojan horse

The Trojan horse presents itself as legitimate software and then allows a hacker to gain access to your computer system. This threat is unusual in that it is usually installed on a computer in good faith. The user genuinely believes they are downloading a useful program. As in the Greek legend that bears its name, the Trojan horse is used to open the doors of a system to other threats such as ransomware. We'll tell you all about them in the next section.

Example of a Trojan horse cyberattack:

3- Ransomware

Ransomware works as follows. Most often, it infects systems using phishing techniques. Specifically, someone clicks on an infected link, opens a corrupted attachment or downloads an item from a dubious site. Once inside the system, the malware wastes no time. It runs undetected and communicates with a remote server to receive instructions.

It then scans all disks for the most sensitive files. Once it has detected them, it encrypts them and blocks their execution. To regain access, the user will have to pay a ransom to the cybercriminal. But of course, there's no guarantee that the cybercriminal will keep his word.

Example of a ransomware attack :

4- Spyware

Spyware monitors and collects your information without your knowledge. It can record your keystrokes, passwords and browsing history, or even capture images of you via webcam. It's therefore a particularly pernicious type of malware, thanks to its discretion.

Typically, it is introduced into a system via freeware from an insecure site. This is why you should always carry out a full analysis of your downloads before installation.

Example of how spyware works:

5- Adware

Adware is best classified as unwanted, rather than dangerous, software.

🎯What's its purpose? To display advertising insistently via pop-ups and redirects to dubious sites with the aim of earning a commission for each click. This won't do too much damage to your system, but it will slow down your browser and put your browsing data at risk.

Example of adware display:

6- Botnet

A botnet is a network of infected machines remotely controlled by a cybercriminal. If your computer is part of a botnet, it becomes a "zombie". In practical terms, this means it can be remotely controlled by the cybercriminal without your knowledge. The aim? To use your computer to carry out massive attacks (such as a DDoS attack), send large-scale spam or mine cryptocurrency.

Botnets are often installed via a Trojan horse or worm. They are particularly formidable, as they do not visibly disrupt your system.

Illustration of a botnet attack:

7- Fake security software (scareware)

As its name suggests, scareware plays on your fear. This malicious software presents itself as an antivirus program, bombarding you with worrying messages about your system's security. The aim is to get you to download a fake premium antivirus that will either have no effect at all (at best), or will be even more damaging. Initially, scareware appears in the form of a pop-up window that's very difficult to close. So it's easy to identify, provided you keep a cool head.

Example of a fake scareware warning message:

8- Fileless malware

Fileless malware does not install itself on the hard disk, but runs directly in RAM. This makes it very difficult for traditional antivirus programs to detect. This type of malware exploits legitimate tools already present on the system, a technique known as Living off the Land (LOTL). Using these tools, the malware can :

execute malicious code in memory,
modify the Windows registry,
elevate its privileges,
propagate laterally across the network.

9- The computer worm

The computer worm is an autonomous piece of malware. It needs no host file to propagate. It duplicates itself and spreads rapidly across a network without the need for human intervention. This makes it extremely dangerous.

In just a few hours, it can paralyze an entire system and inject other malware, exploiting every possible security loophole.

Illustration of the difference between computer viruses and worms: omputer-virus-vs-worm

4 malware case studies

WannaCry

The best-known ransomware is WannaCry. In 2017, it infected over 300,000 computers in 150 countries by exploiting a flaw present in Windows. It was responsible for paralyzing numerous companies and renowned institutions such as FedEx, Renault and several British hospitals.

Emotet

Emotet is a Trojan horse. It is considered one of the world's most dangerous pieces of malware. Spreading via email attachments, it then enabled the installation of other malware such as ransomware. A nightmare for banking institutions.

Pegasus

Pegasus is spyware developed by the Israeli company NSO Group. The spyware has been used extensively by governments to monitor journalists and political opponents. The program is able to take control of smartphones' microphones and cameras without any intervention from the device's owner.

Zeus

Zeus is another Trojan horse that targeted the financial world. Its modus operandi was as follows. Using keylogging software, it captured its victims' banking details. It enabled the theft of hundreds of millions of dollars worldwide, spreading via e-mail and corrupted websites.

How do these malware programs infect a system in just 4 steps?

Step 1: You download an infected application

The great strength of malware is concealment. It hides in the nooks and crannies of files, ready to carry out its misdeeds, or takes the form of reliable software. Real computer chameleons.

An unwary employee can easily download malware in a number of ways.

The most common? A dubious source. The user picks up the software from an unofficial download site, via torrents or on an obscure forum, without realizing the risk. Hence the need for cyber training.

Then there are cracked or pirated versions of paid-for software, usually packed with Trojans.

Finally, fake applications take the form of legitimate software. For example, a fake version of a popular antivirus turns out to be scareware.

Step 2: You click on a booby-trapped link or attachment in an e-mail

Phishing is the main IT threat facing businesses. In 2023, 1.76 billion fraudulent URLs were sent via email. A veritable epidemic.

Phishing works on the principle of social engineering. The cybercriminal sends an email that appears to come from a legitimate source (bank, customer, partner, etc.). The content usually tells the recipient to reset an account to avoid losing it.

And that's how the victim gets tricked. A simple click on a fake contact form, the sending of bank or personal data and the trick is played.

Phishing can also take more elaborate forms. For example, company executives receive much more personalized messages that they think are from their superiors. This is known as whaling.

How can this type of danger be avoided? Create strict zero-trust processes that the whole company must respect.

Step 3: Create vulnerabilities in the operating system

Once malware has been installed, it doesn't stay idle. Its main mission? The malware will first disable your defenses. It neutralizes antivirus software, shuts down automatic updates and disables the Windows firewall. This leaves your computer completely vulnerable. Next, the malware installs backdoors that enable cybercriminals to gain remote access to your machine. These backdoors are particularly vicious, as they remain active even after a reboot.

Step 4: Cybercriminals connect to your compromised network

Cyber criminals now have full access to your IT infrastructure. Thanks to previously installed backdoors, they can discreetly connect to your network. A connection that often goes unnoticed.

Once connected, the hackers proceed to reconnoitre the terrain. They map your network to locate databases, administrator accounts or critical servers, depending on their objectives. Then it's time for the final attack. The hackers will :

encrypt your data (ransomware),
steal your confidential information (data exfiltration),
use your system to launch further attacks (botnet).

What are the consequences of a computer attack?

Loss of files or sensitive data

The most damaging consequence of a malware attack is the destruction of your data. File deletion, database corruption, exfiltration of sensitive information... This is the worst thing that can happen to your business.

The cost is so high (business interruption, recovery costs, damage to reputation) that some organizations never recover.

☝️Prenons the example of Shamoon malware, which hit oil company Saudi Aramco in 2012. This destructive program wiped out the data on over 30,000 computers in just a few hours. The company had to replace its entire IT infrastructure, and it took months to return to normal operation.

Ransom demand (ransomware)

The worst thing about ransomware attacks is their paralyzing effect. Once sensitive files have been encrypted, there's absolutely nothing left to do. Even the world's greatest IT expert is powerless.

There are only two options: pay up or resign yourself to the loss of your files.

💡 We recommend the second option. In fact, paying the ransom offers absolutely no guarantee that the hackers will keep their word. In many cases, moreover, they never restore access.

Secondly, paying cybercriminals for their misdeeds means participating in an immoral economy and promoting an illegal activity that already claims hundreds of thousands of victims every year.

In our expert opinion, the only real solution to ransomware is prevention. Regularly back up your databases on external devices or the cloud. That way, in the event of an attack, hackers won't have a chance.

Long-term monitoring (spyware)

Spyware is the most pernicious form of malware. It doesn't actually destroy your files, but remains discreet and constantly monitors your activity.

Whenever the opportunity arises, it retrieves your passwords, captures your conversations and records your browsing history.

In business terms, it's a formidable weapon for industrial espionage and unfair competition. Your commercial strategies and customer data are at the mercy of hackers who can monetize them by blackmailing you or selling them to your competitors.

Hijacking a company's activities

If, by some misfortune, your company's IT network is integrated into a botnet, the consequences will be unfortunate on two levels.

Firstly, the performance of your installation will be reduced. As your bandwidth is mobilized for a third-party activity, your systems will slow down.

The second problem is legal. If hackers use your structure to commit crimes, you risk prosecution.

Reduced overall IT security

A successful computer attack will weaken your IT security. Malware doesn't just carry out its tasks, it also leaves traces in your system that can facilitate future attacks.

A vicious circle is created: the more your system is compromised, the more vulnerable it becomes to future attacks.

That's why you can't do without a comprehensive cybersecurity strategy.

How can you protect yourself from malware? 5 tips

Use up-to-date antivirus and firewall software

The quickest and easiest thing you can do for your cybersecurity is to install a high-performance antivirus.

This basic protection will provide you with a first barrier against common attacks such as ransomware, Trojans, viruses and so on.

An anti-malware solution also offers other interesting features, such as manual analysis of your system, scanning of your downloads and VPN integration for your browsing.

You can opt for a free solution on a personal level. But if you're a business, we really advise you to opt for a premium package for maximum security.

Avoid suspicious sites and files

Vigilance is the best weapon against many cyberattacks, especially those based on social engineering (phishing, scaraware, etc.).

Before even considering team training, it's essential to implement good IT practices. This means

recognizing red flags, e.g. an email with spelling mistakes, a strange URL, unknown sender, etc. ;
banning dubious download sites and illegal platforms;
implement a strict policy on USB sticks and external media.

Update your operating system regularly

Working on an obsolete software version for several weeks exposes you to unnecessary risks.

So, to avoid fatal oversights, activate automatic updates on all your workstations. Windows, macOS, but also your web browsers, office suites and business software need to be constantly updated.

Pay particular attention to the following applications

PDF readers,
web plug-ins,
compression software.

These are often neglected, even though they are prime entry points for malware.

We strongly recommend that you manage all your company's system updates centrally. You'll have full visibility of the security status of your entire fleet, and updates can be carried out easily, with just a few clicks.

Train employees in cyber risks

According to a study by IBM, over 90% of successful cyber attacks are the result of human error.
A figure that shows just how essential cyber training has become to guarantee corporate security.

👉Every level of the organization needs to be trained in the risks that affect them in particular. For example, management and finance teams need to pay particular attention to the risks of phishing and whaling.

Organize regular practical exercises, such as sending fake e-mails, to identify those members of your organization who require more in-depth training.

Create a genuine culture of cybersecurity, with clear processes to follow in the event of infection and a zero-trust policy.

Back up data securely

Even with an effective cybersecurity strategy, there's no such thing as zero risk. You must therefore have a backup to avoid losing all your data in the event of corruption by a virus or encryption by ransomware.

The best guarantee is a backup. Automate the process to avoid oversights, and make sure that copies are made on a medium external to the system.

Don't forget to regularly test your restoration procedures to avoid unpleasant surprises in the event of an attack.

Which software to use to stop malware?

For effective protection against malware, you need to invest in professional protection software. Simple, free antivirus software is not enough for home users. For businesses and freelancers, 360° security is essential.

We have tested and analyzed numerous anti-malware software products, and here are the references we recommend:

Avast Premium Business Security is a dedicated solution for SMEs with firewall, AI detection of zero-day threats and centralized management platform. Multi-layered protection with a focus on phishing and ransomware risks.
GravityZone Small Business Security (Bitdefender) is modular protection that adapts to your growth. It incorporates advanced machine learning and behavioral analysis technology and a centralized console for all your endpoints.
Malwarebytes is an easy-to-use anti-malware and anti-spyware software aimed primarily at entrepreneurs and SMEs with no technical skills. It offers multi-layered AI protection against malware. Simple and powerful.
ManageEngine EventLog Analyzer is a log monitoring platform for real-time detection of suspicious behavior via AI behavioral analysis.
Norton 360 is an independent/individual multi-purpose software with multi-layer protection, secure VPN and password manager. It features automatic cloud backup and dark web monitoring to keep your personal information safe.