Understanding Trojan Viruses and How to Protect Your Devices

Trojan viruses hide in plain sight. Once activated, they can wreak havoc on networks and devices — some carry keyloggers to steal account and credential data, while others deploy ransomware to encrypt critical files and demand payment for release. 

In this article, we'll break down the basics of Trojan attacks, consider some of the most common types, and explore ways to reduce the risk and impact of Trojan troubles.

What Is a Trojan Virus?

Also called Trojan horses or simply Trojans, this type of malware hides inside seemingly legitimate files and activates without warning. 

Definition of a Trojan virus

A Trojan virus disguises itself as a piece of legitimate software to gain network or device access. In some cases, Trojans are hidden within functioning pieces of software; in others, the software is simply a shell that prevents IT defenses from spotting the Trojan. 

Once a Trojan has compromised a device, network, or application, it activates. This activation may come in response to a specific condition, such as users logging into secure accounts or accessing internal storage devices or may occur after a set period of time.  

History of Trojan viruses

Named after the wooden horse of Greek myth, Trojans appear innocuous on the surface but contain hidden threats. In the myth, the residents of Troy (the Trojans) believed the besieging Greeks had retreated and left the horse behind in their haste. Once the horse was inside the city walls, a force of soldiers hidden inside waited for nightfall then slipped out and opened the city gates. The Greek army returned and conquered the city.  

The first known computer Trojan was detected in 1975. A program called ANIMAL appeared to be a game of 20 questions but also copied itself onto shared directories where it could be found by other users and spread even further. ANIMAL was mostly harmless, as its only function was to replicate. 

In 1989, a DOS Trojan was installed on thousands of floppy disks and mailed out to unsuspecting recipients. Once installed, the Trojan lay dormant until its host computer completed 90 boot cycles. It then encrypted all filenames and asked users to send $189 to a Panama PO box.  

How Trojan viruses work

Trojan viruses work by gaining access to devices under the guise of legitimate software. In most cases, this is accomplished through social engineering. For example, users might receive an email that appears to be from a financial institution or business contact and contains an attachment. Depending on the sophistication of the Trojan, a basic security scan may not detect the malicious code, in turn prompting users to download and install the file.

Once on a system, the Trojan activates in response to user behavior or after a predetermined period.

Common examples of Trojan viruses

Two examples of Trojans in action are the Stuxnet worm and the Emotet Trojan. 

The Stuxnet worm was deployed in 2010 and targeted nuclear weapons facilities in Iran. It did not require an Internet connection but instead spread via a USB drive. The Trojan was designed to attack programmable logic controllers (PLCs) used for uranium centrifuges.   

The Emotet Trojan, meanwhile, was widely used in 2018 to commit financial fraud and theft. Spread through spam and phishing campaigns, Emotet was responsible for $1 million in losses for the city of Allentown, Pennsylvania, and $2 million in damages for Chilean back Consorico. 

Types of Trojan Horse Viruses

While Trojans can contain any type of malware, several types of often seen, including: 

Backdoor Trojans

Backdoor Trojans create a persistent gateway on your network or device that attackers can use to come and go as they please. Even if the Trojan code itself is found and eliminated, the backdoor can remain hidden within other programs or systems for months or years unless a more in-depth scan is performed.  

Banker

Banking Trojans are designed to steal user credentials and compromise financial accounts. The most common form of a banking Trojan masquerades as an urgent email supposedly from your bank or investment company. This message warns that your password has been compromised or your account has been suspended and provides a link to re-enter your credentials and change your password. The link leads to a spoofed webpage that collects your information and passes it on to hackers. 

Ransom Trojan

Ransom Trojans carry ransomware. Once they are inside your system, the ransomware deploys and encrypts some or all of your files. This may render your device virtually unusable or may simply prevent access to key features and functions. Attackers then send a ransom demand, claiming that if you pay, they will provide the decryption key.  

Exploit

Exploit Trojans emerge in response to software or system vulnerabilities. For example, a business might use communications software that contains a known vulnerability. Attackers then create malware capable of exploiting this vulnerability and hide it inside a Trojan. Once deployed, the malware uses the vulnerability to access critical data or compromise network resources.  

Fake AV Trojan

Fake AV Trojans appear as virus warnings. Users visiting compromised websites may receive a warning that their device has been infected with a virus — to fix it, they can pay for protection by clicking a link. If users take the bait, attackers obtain their financial details and can use them to commit further fraud.  

DDoS Trojan 

Dedicated denial-of-service (DDoS) Trojans hide botnets. Once activated, these botnets — effectively zombie computers — make massive numbers of resource and access requests, in turn bringing systems to a halt or crashing them entirely.   

Downloader

Downloader Trojans don't carry any malware. Instead, they carry code that allows them to connect to the Internet and download malicious code. As a result, downloader Trojans may be more difficult to detect. 

Spyware

Spyware Trojans contain software that spies on and records user activities. Spyware may capture data you enter on your keyboard, take screenshots of your applications, or create an inventory of all systems and services running on your device.  

Rootkit

These Trojans carry rootkits, which act to obfuscate malicious objects or processes in your system. Once a rootkit is installed, it can be very difficult to find and remove.  

SMS Trojan

SMS Trojans disguise themselves as standard SMS apps but instead activate without permission to send international messages which may incur a significant cost for device owners.   

Can a Trojan Virus Go Undetected?

The answer is yes. Many can remain undetected for months or years, depending on their evasion tactics and level of sophistication.

For example, the Emotet trojan is a polymorphic Trojan, which means it can modify its code to evade signature-based detection by anti-virus programs.

It’s also virtual machine aware. This means it can detect whether it’s running in a virtual or sandboxed environment and adapt to conceal its presence.

Emotet is both a dropper and downloader, constantly being updated by its operators to bypass traditional defense mechanisms.

For example, it can leverage Dynamic Domain Generation algorithms to create a plethora of fake domains for the malicious Command and Control (C &C) server it’s communicating with. This prevents takedowns of the server.

Emotet can also use various code obfuscation techniques to hide its malicious nature, making it impossible for security solutions to detect its presence.

Other types of trojans that can evade detection include infostealers and (remote access trojans) RATs.

RATs

One of 2024’s most dangerous RATs is Remcos, which allows threat actors to gain remote control of infected systems. Threat actor UAC-0184 primarily uses Remcos to target Ukrainian groups. Meanwhile, North Korean cyberespionage groups have leveraged RATs to attack critical infrastructure assets in Europe and the United States. RATs are often used in advanced persistent threat (APT) attacks and are notoriously difficult to detect.

Infostealers

These trojans infect a device when users unknowingly download malicious attachments from a filesharing site or phishing email. Infostealers can also hide in “cracked software” (a modified version of paid software distributed without proper licensing permissions).Infostealers are notorious for stealing and exfiltrating login data from browsers. 

How to Detect and Remove Trojan Viruses

Trojans succeed when they escape initial notice and deploy without being detected.  

Signs and symptoms of a Trojan virus infection

Given the large number of Trojan virus types, there's no single indicator of Trojan compromise.  There are, however, common signs that accompany many Trojan infections. 

First is strange system behavior — users may find that applications open on their own or refuse to close when asked. Devices may also experience performance problems or have a larger-than-normal number of processes running the background.  

Effective methods to detect Trojan viruses

There are several ways to detect Trojan viruses. 

First are antivirus scanners which actively scan for known Trojan filenames and strange network behavior. Email scanners are another effective method to detect Trojans — by using a cloud-based email scanning service to evaluate every email received, companies can significantly reduce the number of risky messages that end up in user inboxes. 

Human beings also play a key role in detecting Trojan viruses. Put simply, if users notice odd behavior or aren't sure about an email, they should run an AV scan or delete the message. Attackers use social engineering to compromise devices — users can leverage their own social intuition to minimize their risk.  

Steps to remove Trojan viruses from your devices  

Once a Trojan has been detected, it's critical to address the problem ASAP.  

Step 1: Disconnect your device from the Internet.  

This prevents the Trojan from accessing its command-and-control servers or downloading additional malicious files. 

Step 2: Run a full system scan. 

Using a reputable AV tool, scan the system for any infected files and quarantine them. 

Step 3: Delete the infected files.

Permanently delete any infected files, then run another AV scan to ensure nothing was missed. 

Step 4: Update all software and services. 

Update your device to ensure you have the latest versions of all software and services. 

Preventing Trojan Virus Infections

While it's possible to remove Trojans once they're on your device, it's preferable to avoid them entirely. 

Best practices for safe browsing and downloading

Reduce your risk of Trojan attacks by visiting safe websites, such as those with URLs that begin with HTTPS. In addition, don't click unfamiliar links or download unsolicited attachments, and ensure you always log into your accounts using a new browser tab or the company's official application — don't use email or text links.  

Importance of keeping your operating system and software up to date

Keep Trojans outside your device with up-to-date operating systems and software. Updates often contain enhanced security measures or patches for detected vulnerabilities, which reduce the chance of successful Trojan attacks.  

Using reliable antivirus and security software

Solid AV and security software also play a role in Trojan defense. Cloud-based, continually updated security tools can actively scan for potential problems and continually update threat databases. Security tools such as password managers, meanwhile, can limit your risk of compromise even if Trojan attacks break through. 

Understanding the Risks and Damage Caused by Trojan Viruses

According to recent data, Trojans were connected to more than 40% of mobile malware worldwide in Q3 2023. While evolving security frameworks are better equipped to detect Trojans, the social nature of Trojan virus spread means that humans are the weakest link — if attackers can convince users to click on links or download attachments, Trojans can easily infect personal devices or corporate networks. 

Potential risks and consequences of Trojan virus infections

Trojan infections may lead to data loss, financial compromise, or the theft of account credentials. Removing Trojans and remediating systems can be time-consuming and costly, and in some cases, data may never be recovered.  

How Trojan viruses can compromise your personal information

If Trojans obtain login data, they may be able to compromise financial, e-commerce, healthcare, or other personal accounts. From there, attackers can steal your personal data to commit further criminal actions, such as opening credit cards in your name or carrying out tax fraud. 

Impact on system performance and stability

Some Trojan types can negatively impact system performance stability. For example, DDoS Trojans can cause massive slowdowns or network failures, while ransomware Trojans can prevent access to key applications or services.  

Securing Your Devices with LastPass

By securing your devices with LastPass, you're better equipped to keep the gates closed and keep Trojans outside.  

Overview of LastPass password manager

With the LastPass password manager, your credentials are kept safe in your personal LastPass vault. This is because LastPass uses a zero-knowledge model. We don't know the password to your vault, and neither do hackers. It's yours and yours alone. This means that even if attackers manage to compromise some of your data, they cannot access your password data.  

How LastPass can help protect against Trojan viruses

Trojan viruses often target user credentials. With LastPass, users can enable full endpoint protection, which requires the use of one-time passcodes or fingerprint scans in addition to login details, in turn rendering stolen credentials useless.  

Features and benefits of LastPass for enhanced security

LastPass provides multiple benefits for enhanced security. 

Best-in-class encryption

Your master password and stored password are kept secret, even from LastPass. Your vault is only ever encrypted or decrypted at the device level. 

Global security certifications

LastPass holds security certifications including ISO 27001, SOC Type II, SOC3, and TRUSTe to ensure compliance.  

Protection against the dark web

LastPass both protects your private data and notifies you if it is compromised or appears on the dark web.  

Trojan viruses present significant risks for individual users and business operations. Keep these deceptive decoys outside the perimeter where they belong by recognizing the signs of compromise, taking action to reduce risk, and implementing enhanced security solutions to protect passwords and personal data. 

See how LastPass keeps your data safe. Start your free LastPass trial today.