What is a Trojan?

What is a Trojan: Understanding Trojan horse malware

Trojan horse malware is named after the ancient Greek story in which soldiers hid inside a wooden horse to sneak into the city of Troy. In cybersecurity, Trojans are a type of malicious program that uses the same tactic: it disguises itself as a harmless or helpful file to trick users into letting it into their systems. The term 'trojan horse virus' is often used to describe this type of malware, though technically, Trojans are different from computer viruses. Unlike computer viruses, Trojans do not self-replicate but can be just as damaging. Trojans often operate without the user's knowledge, making them particularly dangerous. Once installed, the malware can cause significant damage.

This article explains how Trojan horse malware works, what it can do, how to recognize it, and how to protect yourself from infection.

Introduction to malware

Malware, short for malicious software, is any type of software created with the intent to damage, disrupt, or exploit computer systems and networks. Among the many types of malware, the Trojan horse stands out for its deceptive tactics. A Trojan horse, sometimes called a Trojan virus, is a type of malware that pretends to be legitimate software, tricking users into installing it on their devices. Once inside, a Trojan can steal sensitive data, gain backdoor access to the system, and install additional malware without the user’s knowledge. This makes Trojan horses a serious threat to computer security across all major operating systems, including Windows, macOS, and Android. Understanding how this type of malware operates is crucial for anyone looking to protect their sensitive data and prevent unauthorized backdoor access to their devices.

What is a Trojan horse?

A Trojan horse, also known as Trojan malware, is a type of malicious software that pretends to be legitimate. It is designed to trick users into installing it on their devices. Trojans often disguise themselves as other files, such as documents or installers, to trick users into opening them. Once active, it can perform a range of harmful actions, including stealing data, giving attackers remote access, or installing additional malware.

Although many people refer to it as a “Trojan virus,” this is technically incorrect. A virus replicates and spreads by itself. A Trojan cannot self-replicate; it needs the user to install it. That’s why the correct term is “Trojan malware” or “Trojan horse malware.”

If you’d like to learn more about malware and its effects, check out our guide on what is malware.

The history and impact of Trojan attacks

Trojan malware has existed since the early days of computing. The first known Trojan dates back to 1971, but it became a major threat during the 1980s as personal computing became more widespread.

Today, Trojans are responsible for:

Widespread data breaches
Financial fraud and stolen banking credentials
Damaged or destroyed files
Backdoor access to corporate and personal systems
Silent surveillance of user activity
Secondary malware infections
Targeting victims' computers to steal sensitive information or facilitate further infections
Identity theft as a consequence of stolen personal and financial data
Stealing account data, such as login credentials for online banking and e-payment systems

Trojans are also commonly used in online scams to trick users into installing malicious software.

Trojans account for a large share of global malware infections, making them a persistent threat.

How Trojan malware works

Trojan malware does not spread by itself. Instead, it relies on human interaction. Users are tricked into downloading and executing the malware because it appears legitimate or useful.

Common infection methods include:

Phishing emails with infected attachments
Clicking on malicious links that lead to fake websites
Downloading pirated or fake software
Visiting compromised websites
Interacting with pop-up ads that contain hidden malware

Once installed, the Trojan begins running in the background and may operate silently for extended periods. It can provide attackers with unauthorized remote access to the user's computer, putting personal and system security at risk. The Trojan may also execute various malicious tasks, such as stealing data, damaging files, or causing system disruptions.

What can a Trojan horse do?

Trojan horse malware can be extremely dangerous. Depending on how it is designed, it may be able to:

Steal sensitive information such as passwords or banking details
Record your keystrokes using keyloggers or capture keystrokes to steal sensitive information
Steal email addresses from the infected device
Target user credentials and user account information, especially for online services
Steal account data and access data, such as login details for banking and e-payment systems
Steal data from the device
Encrypt, modify, or delete important files
Monitor your activity or access your webcam and microphone
Install other types of malware
Open a backdoor for remote access by cybercriminals
Slow down your system by running background processes
Bypass antivirus software and remain undetected

Infostealer trojans are a type of Trojan specifically designed to steal personal and financial information. Spy trojan programs can monitor user activity and collect sensitive data. Trojans are a type of malicious programs that can perform a wide range of harmful actions.

Trojans are often part of larger attacks and may be used to support ransomware, botnets, or data theft.

Is a Trojan a virus?

No, a Trojan is not a virus. A virus can replicate and spread from one device to another without user involvement. A Trojan, by contrast, requires user action to be installed. It does not spread automatically or infect other systems on its own. The term "trojan horse viruses" is sometimes used to describe these threats, but Trojans differ from traditional viruses in how they operate and spread.

This is why the term “Trojan virus” is misleading. The correct term is “Trojan malware” or “Trojan horse.” Trojans can also be used to deliver other malware, such as ransomware or spyware, to the infected system.

How Trojans infect a device

Trojans infect devices through deception. Users are misled into believing a file is safe, which leads them to install the malware.

Typical infection methods include:

Opening infected email attachments
Downloading software from untrusted or unofficial sources
Downloading or installing Android malware disguised as legitimate apps, which can infect mobile devices with SMS Trojans
Clicking on links in phishing messages
Visiting websites that contain hidden malware
Clicking on fake ads, pop-ups, or fake antivirus alerts
Using outdated software with known vulnerabilities

In almost all cases, the infection depends on the user being tricked.

Common types of Trojan malware

Trojans are a type of malicious programs with a variety of purposes. There are many types of Trojan horse malware, each with a different purpose. Common examples include:

Infostealer Trojans: Designed to steal personal and financial information, playing a major role in cybercrime activities like identity theft and data theft.
Spy Trojans: Monitor user activity, collect sensitive data such as keystrokes and screenshots, and record passwords.
Backdoor Trojans: Give attackers remote control over the infected device, can be remotely updated to evade detection, and may recruit the device into a zombie network.
Banking Trojans: Designed to steal login credentials for online banking systems and e payment systems, targeting online financial transactions.
Downloader Trojans: Download and install other types of malware.
Exploit Trojans: Target known software vulnerabilities to gain access.
Fake Antivirus Trojans: Pretend to be security tools and trick users into installing them.
Ransomware Trojans: Encrypt files and demand payment for their release.
Rootkit Trojans: Help malware stay hidden from antivirus software.
Trojan-IM: Target messaging platforms to steal login credentials or read messages.
Mobile Trojans: Infect mobile phones, often disguised as legitimate apps, and may send unauthorized text messages to compromise security.
Zombie Computer Trojans: Turn infected devices into zombie computers, which are secretly controlled as part of a zombie network to carry out distributed denial of service (DDoS) attacks and other malicious activities. Learn more about DDoS attacks in our guide on what is a DDoS attack.

Some Trojans, such as Skygofree, are specifically designed to target messaging services and track user activity.

Well-known Trojan malware examples

Several Trojan malware programs have caused widespread damage. Some of the most notable include:

Zeus: A banking Trojan used to steal login information and financial data
Stuxnet: A sophisticated Trojan that targeted industrial control systems
Gameover Zeus and Dyre: Distributed through phishing and credential theft campaigns
TeslaCrypt, Locky, and WannaCry: Combined ransomware functions with Trojan-like behavior

These Trojans have shown how powerful and destructive malware can be when it relies on deception and social engineering. Once a Trojan infects a computer, it can compromise the security of the infected computer and, in some cases, spread across computer networks, putting entire systems at risk.

Instant messaging threats

Instant messaging platforms have become a popular target for Trojan horse attacks. Classic services like ICQ, MSN Messenger, AOL Instant Messenger, and Skype, as well as modern apps such as Facebook Messenger, WhatsApp, Telegram, and Signal, can all be exploited by cybercriminals. Trojan-IM programs are designed to steal login data and passwords, giving attackers the ability to access user accounts and sensitive conversations. In some cases, attackers have even used instant messaging channels to control Trojan malware remotely, as seen in December 2020 when a Windows Trojan was managed via a Telegram channel. To defend against these threats, users should be vigilant about phishing attacks and avoid clicking suspicious links or downloading unknown files through instant messaging. Using robust antivirus and antimalware software adds an essential layer of protection, helping to detect and block Trojan horse malware before it can compromise your accounts.

For tips on staying safe, learn more about what phishing is and how to avoid it.

Email attachments

Email attachments remain one of the most common ways for Trojan viruses to infect computers. Cybercriminals often disguise malicious attachments as important documents or files from trusted contacts, luring users into opening them. Once the attachment is downloaded and opened, the Trojan malware installs itself on the infected device, often running automatically every time the system starts. In addition to email attachments, Trojan viruses can also be hidden in banner ads, pop-up advertisements, or links on compromised websites. To reduce the risk of infection, users should be cautious when opening emails from unknown senders and avoid downloading attachments from untrusted sources. Keeping your operating system and antivirus software up to date is essential, as is using a reputable antivirus program to scan all email attachments for malicious code. These steps can help prevent Trojan malware from gaining a foothold on your device and protect your sensitive data from being compromised.

Signs of a Trojan infection

Although Trojans are designed to remain hidden and often operate without the user's knowledge, there are several signs that your system may be infected:

System performance is unusually slow
Applications crash or behave abnormally
New programs appear without your knowledge
Your homepage or browser settings change unexpectedly
Pop-up ads appear frequently
You are redirected to unfamiliar or suspicious websites
Fake antivirus warnings or update messages appear

If you experience any of these issues, you should run a full malware scan using trusted antivirus software.

How to remove a Trojan

If you suspect a Trojan infection, take action immediately to clean your infected computer and restore its security.

Recommended steps:

Disconnect from the internet to block remote access and data transmission
Restart your computer in Safe Mode to limit the malware’s activity
Review installed programs and remove anything unfamiliar or suspicious
Run a full system scan using up-to-date antivirus or anti-malware software
Follow the software’s recommendations to quarantine or delete the malware
Change passwords for all your online accounts
Monitor your accounts and systems for further signs of compromise

In severe cases, a full operating system reinstall may be necessary.

How to prevent Trojan infections

Preventing a Trojan infection is far easier than dealing with one. Effective prevention strategies include:

Keep your operating system and all applications up to date
Use reputable antivirus and anti-malware tools with real-time protection
Avoid opening attachments or clicking on links in emails from unknown sources
Download software only from trusted websites or official app stores
Avoid pirated software, which is often bundled with malware
Be cautious with pop-up ads or suspicious offers online
Enable your firewall and use browser security features
Scan USB drives and external media before opening their contents

Regular maintenance and cautious behavior are key to reducing your risk.

The importance of antivirus software

Antivirus software plays a critical role in detecting, blocking, and removing Trojan malware. A strong security solution can:

Scan and remove known threats
Monitor system behavior for suspicious activity
Block access to dangerous websites
Warn you about malicious downloads or fake applications
Automatically update to recognize the latest threats

Choosing reliable, well-supported antivirus software is an essential part of any cybersecurity strategy.

The role of user awareness

Many Trojan infections happen because users are unaware of the risks. That is why cybersecurity education is so important. Security researchers play a crucial role in uncovering new Trojan threats and educating the public about cybersecurity risks.

Users should learn how to:

Recognize phishing attempts and social engineering tactics
Avoid downloading unknown files or clicking suspicious links
Stay informed about current threats and scams
Report unusual activity to IT or security teams promptly

Organizations can greatly reduce the risk of infection by training staff regularly and enforcing clear security policies.

What to do if you are infected

If you believe your system is infected by a Trojan:

Disconnect from the internet to prevent further damage
Scan your system with antivirus software
Remove the malware or follow quarantine procedures
Change all passwords, especially for accounts related to email, banking, or cloud storage
Notify your bank or credit card company if financial data may have been exposed
Consider reinstalling your system if the infection is severe
For businesses, follow your incident response plan and document the event

A quick and coordinated response can limit the consequences of a Trojan attack.

Network security against Trojans

For organizations, network security is essential for preventing and containing Trojan infections and protecting computer networks from being compromised. Best practices include:

Installing antivirus solutions across all endpoints
Using firewalls and intrusion detection systems to monitor network activity
Ensuring that software updates and security patches are applied promptly
Segmenting the network to limit the spread of infections
Performing regular security audits and penetration tests
Restricting software downloads to trusted sources only

These measures help create multiple layers of defense that make it harder for Trojans to succeed.

Conclusion

Trojan horse malware remains one of the most common and dangerous types of cyberthreats. Its ability to disguise itself and exploit human error makes it difficult to detect and stop without the right tools and knowledge.

Combining cybersecurity awareness, trusted antivirus software, and strong security policies can significantly reduce the risk of infection. Whether you are an individual user or managing a business network, understanding how Trojan malware works is essential to protecting your systems and data.