What Is a Trojan Infection and How to Protect Against It in 2024 by Katarina Glamoslija

Katarina Glamoslija

Updated on: June 11, 2024
Lead Cybersecurity Editor

Remember the old tale from the Trojan War? When the Greeks left behind an enormous wooden horse and pretended to abandon the Siege of Troy and when the Trojans took it inside, the soldiers hidden inside opened the gates to the Greek army and conquered the city?

Well, like the wooden horse, Trojan viruses disguise themselves as something else to gain access to your computer or device. Trojans might use the same file name as a legitimate app so that you’re tricked into inviting them in. The hackers controlling the Trojans can then access, modify or delete your data, as well as disrupt the performance of your computer or your computer network.

Most Trojans aim to take control of your computer and steal your sensitive data while acting as a gateway to allow even more malware to infiltrate. So how do you quickly spot the telltale signs of a Trojan infection before it’s too late and defend your PC and devices against these malicious attacks?

We’ll show you.

What Do Trojans Do?

There are many different types of Trojans, and they can do many different things. Once inside your device, a Trojan can lay low, collecting information and setting up holes or backdoors into your system undetected. On the other hand, it may just take over your computer.

There are many reasons for hackers to use Trojans. Some of the main uses are to:

• Steal and Modify Data. Trojans are capable of accessing, altering, and deleting data. This is especially worrisome if you store sensitive personal or business information on your device. A Trojan is often designed to wait patiently in the background until you access your banking app or enter your credit card details. It can also be used to collect data, such as recording the keystrokes and browsing history of the sites you visit, as well as your login details and passwords for websites and banking apps. Then it sends that information on to the hacker.
• Interrupt the Regular Performance of the System. Sometimes a hacker isn’t interested in your data but wants to use your computer to stage a massive attack on another system or to mine cryptocurrency. With the rise in cryptocurrencies, cryptojacking has become an extremely profitable activity for hackers and is rapidly growing.
• Install More Malware and Create Backdoors. Once inside your device, Trojans will often make changes to your security system, leaving a backdoor for a hacker to use later. It will often download and install other malware onto your device, making it vulnerable to viruses and worms.

In a nutshell, each Trojan is created with a specific job in mind, including any of the following:

• Intercepting passwords and personal details.
• Stealing bank details and credit card information.
• Gaining control of your computer.
• Installing other types of malware.

Types of Trojan Horses

While Trojans are often labeled as viruses, this definition is not technically correct. A computer virus will attempt to spread the infection wherever possible, whereas a Trojan is an individual program with a specific task, such as any of the following:

• Rootkit — works by undermining your computer’s system activities. It allows malware to run undetected, to increase the length of time and amount of damage that can be achieved by one infection.
• Backdoor — grants complete remote control to the operator, allowing them to edit, send, download, and delete files. They are often used to hijack personal devices for criminal activity.
• Exploit — takes advantage of a security loophole within your computer software. Whether in a specific app or affecting the operating system itself, they can manipulate a vulnerability to gain direct access to your files.
• DDoS — solicits your computer to send countless requests to a specific URL, with the aim of overloading the server and shutting down the site.
• Spyware — aims to intercept your personal information. This goal is achieved by copying your files or using a screen or keylogger to record what you type and which websites you visit.
• Ransomware — once the malware is on your computer, it locks you out of specific areas. The only way to regain access is by paying a ransom.
• Banker Trojans — redirect you to fake websites or intercept your transactions to steal your financial info, such as account numbers, online banking credentials, and credit card details.
• SMS Trojans — target mobile devices and send text messages to premium-rate numbers, resulting in charges which are then transferred to the cybercriminal.

While these are all common types of Trojans, their goals vary considerably. Most aim to steal information for financial gain. This can include bank information, online gaming details, IM log-ins, personal contacts, phone data, and more.

Famous Examples of Trojans

• Zeus: Zeus is an infamous banking Trojan which was first detected in 2007. It targeted banking information through keystroke logging and form grabbing, and was primarily spread through phishing schemes. Zeus infected thousands of businesses and individual users, leading to huge financial losses.
• Emotet: Emotet was originally a banking Trojan, but it has evolved into a more versatile threat, serving as a delivery vehicle for other types of malware too. It can spread rapidly through networks, making it one of the most widespread and persistent malware threats.
• Stuxnet: Stuxnet was designed to target Iran’s nuclear facilities — it specifically aimed to sabotage industrial control systems. Stuxnet was able to manipulate centrifuges in nuclear reactors to spin out of control while displaying normal operating conditions, which caused significant disruptions.
• CryptoLocker: CryptoLocker was a ransomware Trojan that encrypted users’ files and demanded a payment to restore access. It was highly successful, infecting several million computers, and its method of using a cryptographic key known only to the attacker set a precedent for various future attacks.

How to Spot Trojans

Trojans can look like just about anything: a computer game you downloaded a month ago, a Twitter link you clicked on that led you to a strange website, or an email attachment you downloaded that was sent from an old friend’s email address.

Unlike viruses, Trojans can’t spread on their own. Instead, they survive by staying unnoticed and hiding behind files and programs you think you can trust. Trojans rely on you to run them by mistake or through negligence. If you fall victim, you may not even realize it until it’s too late.

Look out for the following four signs, which may be a warning of Trojans attacking your computer:

• Unexplained Behavior. Any mysterious increase in CPU usage is definitely a red flag. If your computer has increased its processing activity for no reason, then a Trojan may be the reason. Use your activity monitor to check what is draining your CPU, and end the action if you find a problem. You can Google any processes you don’t recognize to determine whether they’re malicious or just part of normal system activity.
• System Failures. If your system suddenly slows down significantly or starts crashing regularly, then there’s something wrong. Use a high-quality security suite to see if you can identify the problem.
• Increase in Spam. Pop-ups, annoying adverts, and a general rise in spam on your machine could indicate an adware Trojan. This malware type uses infected ads to spread the virus further. Avoid clicking on anything that looks suspicious!
• Unidentified Programs. If you spot an app or program that you didn’t download, it’s a cause for concern. Google the name to ensure it’s not an important part of the operating system. If it’s not— delete it immediately!

What to Do if You Have a Trojan Infection

If you suspect you have a Trojan on your device, the first thing you should do is disconnect it from the internet. This will prevent the Trojan from sending additional data to a malicious actor or downloading further malware.

Next, reboot your computer in safe mode to prevent the Trojan from loading. Typically, you can do this by pressing the F8 key as your computer starts up.

Now, you should run a full scan of your device with a reliable antivirus. I’m using Norton in this example.

Don’t stop the scan before it completes — always let it finish because there might be more hidden threats that the antivirus is yet to find.

Once the scan is complete, you will see a list of quarantined files. You can now safely remove these.

It’s a good idea to restart your computer at this point and run another scan, to make extra sure that every threat has been eliminated.

Your computer is now Trojan-free — however, you should definitely change any important passwords in case they’ve been intercepted while the Trojan was active on your device. Then, make sure to follow our safety tips below.

Tips to Avoid Getting Infected

Trojans take advantage of the weakest link in your computer’s defenses: you.

These forms of malware are carefully crafted to trick the user into downloading a harmful file. By carefully disguising the malware as something you might want to download, hackers try to trick you into downloading it.

Trojans are everywhere — in emails, on social media, and on the web. By clicking on a clickbait ad or checking out an attractive offer online, your device can be infected with a Trojan without you even noticing.

While not all hackers may be able to hack into the Pentagon, they understand human behavior very well. No matter how careful you are, it is almost impossible to avoid malicious links and files.

That said, there are ways to prevent contracting a Trojan infection:

• Be wary of email attachments. Trojans are often sent via email attachments in spam emails. By downloading the file, you actively infect yourself. Avoid opening any emails from people you don’t know. Even if you trust the sender, check with them that they intended to send the attachment, as they may be the victim of a hack.
• Approach freeware with caution. Freeware is no-cost software that is easy to download. While these free programs are convenient, Trojan horses are known to piggyback on them. Do some independent research and read reviews on any free program you’re thinking about installing before downloading it.
• Install a high-quality antivirus. Antivirus and anti-malware software should be your first line of defense — and it’s essential to install something robust enough for the job. Quality antivirus programs like Norton scan your device for problems and alert you if an issue arises. They can also quarantine and delete any threats from your device.
• Avoid third-party downloads. Any download is a potential threat. Websites, ads, and messages that contain automatic downloads often hide malware. Avoid clicking through to any banners and suspicious links, don’t use shortened URLs, and think carefully before allowing any download.
• Stick to trusted sources. Whether it’s a website, correspondence, or software, always stick to trusted brands. If companies are tried and tested by the general public, you’ll know if any problems have arisen. Reputation is king on the internet so always check independent reviews as well.
• Use a firewall. A firewall filters the data that comes into your device from the internet. Although most operating systems include a built-in firewall, implementing a customizable firewall like the one that comes with Bitdefender can provide an additional layer of security and complete protection.

Trojans are a complex form of malware, so you need more than two eyes to spot them. Using security software like antivirus, anti-malware, and firewalls will give you an extra layer of protection.

Frequently Asked Questions

What is a Trojan infection?

A Trojan is malicious software that disguises itself as legitimate to deceive you into loading and executing it on your computer. Unlike viruses, Trojans don’t replicate themselves, but they can perform a range of harmful acts like stealing data or creating backdoors for hackers.

Is a Trojan a virus?

No, a Trojan isn’t a virus (though it’s common for people to refer to Trojans as viruses). While both are types of malware, a virus replicates itself and spreads, infecting other files. In contrast, a Trojan deceives users by appearing benign or useful but performs malicious activities once it’s executed.

Can Trojan infections be removed?

Yes, Trojan infections can be removed using antivirus software. Quality antivirus programs like Norton can scan your computer, identify the malicious software, and safely remove it. You should keep your antivirus software updated and running regular scans — this is crucial for effective removal and prevention of future infections.

Can Trojans affect mobile devices too?

Yes, Trojans can also affect mobile devices. They often appear as legitimate apps and, once installed, can compromise the device by stealing personal information, sending SMS messages to premium-rate numbers, or installing other malicious apps. Remember to always download apps from reputable sources and keep security software updated to guard against these threats.