What is a Trojan horse virus?
After a fruitless ten-year siege, the Greeks resorted to building a large wooden horse, filling it with the elite force, and leaving it in front of the city walls, hoping to deceive the Trojans into wheeling it in. The Greek plan was successful—Trojans pulled the wooden horse in as a victory trophy. Little did they know that Greek soldiers would creep out of the horse during the night, open the gates for the rest of the army, destroy Troy, and end the war.
Yet this Trojan horse mythological story doesn’t end there. Today, the term “Trojan horse” refers to a type of malware that appears to be a legitimate program but, at its core, contains hidden malicious code. So, without further ado, let’s learn what a Trojan horse virus is, how it works, and how to prevent such attacks.
Trojan horse virus definition
A Trojan horse is a type of malware that disguises itself as a legitimate program, file, or email attachment to trick users into downloading and executing it. Once Trojan malware is inside your computer, it can perform its destructive actions.
How does a Trojan horse work?
Similarly to the Greeks, Trojan horse malware's success depends on its victim being tricked into downloading and installing the cloaked virus as a program themselves. Then, some Trojan horse viruses unleash malicious acts instantly, while others await further instructions from their host cybercriminal.
Most Trojan horse viruses spread through files attached to legitimate-looking emails. But that’s not all. Trojan viruses also lurk on file-sharing websites, such as torrent sites or sites allowing users to share their files. Although these sites are attractive to many because you can get software, games, or music files without paying the retail price, they are also attractive to cybercriminals who are looking for a way to access your computer.
By now, you can guess that Trojan horses can take many shapes and forms. Another one is message spoofing, where the attacker disguises a Trojan virus as a legitimate message or alert to trick users into downloading or installing the malware. This can be a message from a trusted source, a legitimate software update, or even, unsettlingly, an antivirus alert.
But Trojan attacks don’t just stop at targeting individual users. Some attackers will find weaknesses in unsecured websites where they will upload malicious files or even take over the entire site. Once hijacked, the bad actor can redirect users who wander into the said website to other sites or redirect your downloads to a malicious server containing the Trojan virus.
Hacking Wi-Fi networks is also a common way to spread Trojan viruses and other malware. By creating a fake hotspot network that looks like the real one you’re trying to connect, a cybercriminal can redirect you to fake websites that, also, look extremely real. In this distorted reality, these fake sites that contain browser exploits can redirect you to any file you try downloading.
Once the most innocent email attachment or link containing a malicious virus is opened, the Trojan malware will install and automatically run every time the device is turned on.
Types of Trojan viruses
Knowing which type of Trojan virus entered your system will allow you to mitigate the risk in the future. There are many types of Trojan horse viruses, each with a specific purpose and attack methods. So here is a list of the most common ones:
Backdoor Trojans
Creates a backdoor for attackersto access and even control your device remotely. This allows them to upload, download, or execute files whenever they want. Backdoor Trojans are used for various malicious activities, such as deleting files, stealing data, manipulating systems, or launching other attacks.
Downloader Trojans
Downloads and installs more malicious programs on your computer, such as ransomware, adware, or other malware. This type of Trojan virus may be challenging to detect and remove, so it’s used to bypass security software.
Ransomware Trojans
Encrypts files on your computer and demands a ransom to unlock them. This can cause significant damage to the company’s finances and reputation.
Distributed Denial-of-Service (DDoS) Trojan
Launches DDoS attacks that overload a network with traffic by sending multiple requests to target systems and making them unavailable. This Trojan virus often turns infected devices into botnets, compromised computer networks controlled by an attacker to launch large-scale DDoS attacks.
Banker Trojans
Targets information related to banking and online transactions. It often uses keyloggers to capture sensitive information such as credentials, passwords or credit card details as it enters into the computer.
Rootkit Trojans
Hides other malware on an infected operating system by concealing themselves from antivirus software and making it difficult to detect and remove other malicious code. This Trojan horse allows attackers to access the system and perform various harmful actions.
Spy Trojans or Spyware
Spies on and logs the victim’s actions on their computer. This includes keylogging, taking screenshots, and collecting sensitive data like account credentials or banking details. Once collected, the Trojan sends this data to the attacker. This Trojan horse is typically disguised as useful software.
How to identify a Trojan malware attack
Trojan horses can replicate themselves across a network and download data that can later be leaked or used against you or your company. This sounds scary, especially when you add that Trojan horse attacks are hard to notice or remove. Yet, there are things you or your cybersecurity teams can look out for to tackle any type of Trojan horse:
- The first sign something may be wrong and a Trojan virus might have hit you is that an opened document or program doesn’t look or work as it should.
- Your device’s performance becomes slower, more unpredictable, and unreliable than usual.
- Anytime you use your browser or operating system, you are interrupted by unsolicited pop-ups, notifications, and even spam.
- You start to notice unexplained procedures, processes, or programs that started to run from your device.
Once you notice these disruptions, there’s a high chance you’ve been hit by a Trojan horse attack and should start working on mitigating the risks.
Real world examples of the Trojan virus attacks
There are at least several Trojan horse attack names that echoed throughout the world’s media, so let’s review three of them here:
Emotet
This Trojan horse virus first shook the cybersecurity landscape in 2014, and till now, it’s considered one of the most persistent and adaptable threats. It can evade standard antivirus detection, making it a persistent threat.
Emotet’s initial version was a banking Trojan virus aimed at stealing banking credentials from infected devices. Yet, soon, its impact extended beyond financial losses and posed a threat to public sector operations by compromising the security of government systems.
The Emotet malicious code infects the victim’s computer via email attachments presented as invoices, shipping notices, and data about COVID-19. All these emails contained malicious Word documents, either attached to the email or given as a link within the email.
Zeus
Trojan horse, then Zeus—the malware world indeed has a taste for Greek mythology. Zeus is a Trojan horse malware package that was designed to steal personal financial data. It became widespread in March 2009. The following summer, it compromised over 74,000 FTP accounts on Bank of America, NASA, ABS, Amazon, and other websites.
This virus connects devices to a botnet and then uses it to steal money from major corporations and banks. Even with up-to-date antivirus and other security software, it’s complicated to detect as it successfully hides with stealth techniques.
ILOVEYOU
Disguising as an affectionate sentiment, this Trojan virus reached its victims via email that rapidly infected ten million Windows computers globally in 2000.
When users opened the attachment labeled "ILOVEYOU"—which appeared to be a text file, the Trojan virus would overwrite random files and send itself to the victim’s entire contact list to infect other operating systems.
How to prevent Trojan horse attacks
As always with cybersecurity, companies should invest in both employee awareness and the tools their security teams use to mitigate Trojan horse attacks.
Firstly, regular cybersecurity training should be scheduled to cover, among other things, Trojan horse attacks and how they enter devices and operational systems. These trainings should teach that each employee should always have an active antivirus protection on and follow best practices to avoid this virus infections, namely to never:
- Download software from untrusted sites.
- Open email attachments if it looks suspicious.
- Run programs from unrecognized or suspicious emails.
Security teams should ensure that all operating systems and applications are up to date, minimizing the overall vulnerabilities across the network. Trojan malware can enter a network and remain inactive for extended periods, so security teams should periodically scan networks and devices to identify potential threats before they can spread and breach your company’s security.
That’s why it’s important to invest in high-quality security tools. Security teams should install and use trusted antivirus software that is on the front lines of spotting and removing Trojan viruses. Antivirus solutions will search and detect trojan signatures in files, isolate them, and promptly remove them. They can also detect suspicious activity inside any applications on your devices.
For extra security, you can use the NordLayer Download Protection feature, which automatically scans every file upon download. This can protect your company’s assets in real-time from malware and ensure that every new download is scanned and checked. If malware is detected, it will be removed instantly, and it will not disrupt or slow down work.
Final thoughts
Like the original myth, the digital version of it is a sneaky gift that you really don't want to introduce to your computer. Even worse, they can infect your device and cause considerable problems before you even understand what happened. Once it's on your computer, it can install additional malware or monitor your keyboard.
This attack relies on human error—on users opening a harmless file, such as an email that says ILOVEYOU. So stay vigilant, invest in your employees' cybersecurity training and security tools, and you will be able to leave the Trojan horse outside your network walls.