Trojan horse

The term Trojan Horse refers to a type of malicious software (malware) that deceives users into running it by pretending to be a harmless or beneficial program.

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The term Trojan Horse refers to a type of malicious software (malware) that deceives users into running it by pretending to be a harmless or beneficial program. Named after the mythical Greek story, the Trojan Horse in cybersecurity is a deceptive tool used by cybercriminals to infiltrate and exploit computer systems.

Understanding the concept of a Trojan Horse is crucial in today's digital age, where cyber threats are increasingly sophisticated and prevalent. This glossary entry aims to provide a comprehensive understanding of what a Trojan Horse is, how it operates, its types, and how to protect against it.

Origins of the term

The term Trojan Horse in cybersecurity draws its roots from the ancient Greek story of the Trojan War. In this tale, the Greeks, unable to penetrate the fortified city of Troy, hid their soldiers inside a giant wooden horse and presented it as a peace offering. The Trojans, believing the horse to be a gift, brought it into their city. Once inside, the Greek soldiers emerged from the horse, opening the city gates for their army and ultimately leading to the downfall of Troy.

Similarly, in the digital world, a Trojan Horse is a malicious program that disguises itself as a useful or harmless application. Once downloaded and installed by an unsuspecting user, it can cause significant harm by stealing sensitive information, damaging files, or allowing remote control of the user's system by a hacker.

First known Trojan Horse

The first known Trojan Horse in computing history was a program called 'ANIMAL', created by John Walker in 1975. ANIMAL was a game that asked users to guess the type of animal it was thinking of. However, while the game was running, it would also replicate itself in the background, copying its code into other directories.

Although ANIMAL was not malicious, it demonstrated the potential for programs to perform hidden actions without the user's knowledge or consent, paving the way for the development of malicious Trojans.

How trojan horses work

Unlike computer viruses, Trojan Horses do not replicate themselves. Instead, they rely on social engineering techniques to trick users into downloading and installing them. A Trojan Horse might be disguised as a free game, a software update, an email attachment, or even a link sent through a social media message.

Once installed, a Trojan Horse can perform a variety of malicious activities, depending on its type and purpose. It might steal personal information, such as credit card numbers or passwords, damage or delete files, install other malware, or create a 'backdoor' that allows a hacker to remotely control the infected system.

Stages of a trojan horse attack

A Trojan Horse attack typically involves three stages: delivery, installation, and execution. During the delivery stage, the Trojan Horse is transmitted to the target system. This could be through a malicious email attachment, a compromised website, or a fake software update.

During the installation stage, the Trojan Horse is installed on the target system. This often requires the user to take some action, such as opening an email attachment, clicking on a link, or agreeing to a software update. Finally, during the execution stage, the Trojan Horse carries out its malicious activities. This could involve stealing information, damaging files, or creating a backdoor for remote access.

Types of trojan horses

There are many types of Trojan Horses, each designed to perform specific malicious activities. Some of the most common types include the Backdoor Trojan, the Infostealer Trojan, the Ransom Trojan, the Remote Access Trojan (RAT), and the Distributed Denial of Service (DDoS) Trojan.

Backdoor Trojans create a 'backdoor' into the infected system, allowing the attacker to gain remote control. Infostealer Trojans are designed to steal information from the infected system. Ransom Trojans encrypt the user's files and demand a ransom for their release. RATs allow the attacker to remotely control the infected system, while DDoS Trojans are used to carry out distributed denial of service attacks, overwhelming a target system with traffic to make it inaccessible.

Evolution of trojan horses

Over the years, Trojan Horses have evolved to become more sophisticated and harder to detect. Early Trojans were relatively simple and could often be detected by antivirus software. However, modern Trojans use advanced techniques such as rootkits and polymorphic code to evade detection.

Rootkits are tools that allow malware to hide deep within the operating system, making it difficult for antivirus software to find and remove them. Polymorphic code is code that changes each time it runs, making it hard for antivirus software to recognize the Trojan as malicious.

Protection against trojan horse

Protecting against Trojan Horses involves a combination of good cybersecurity practices and the use of reliable security software. Users should be wary of downloading software or opening email attachments from unknown sources, and should keep their operating system and applications up to date to protect against known vulnerabilities.

Security software, such as antivirus and anti-malware programs, can help detect and remove Trojan Horses. However, because Trojans are constantly evolving, it's important to keep your security software updated with the latest virus definitions.

Role of firewalls

Firewalls play a crucial role in protecting against Trojan Horses. A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It establishes a barrier between a trusted internal network and untrusted external networks, such as the Internet.

By blocking unauthorized access and allowing legitimate traffic to pass, firewalls can help prevent Trojan Horses from reaching your system in the first place. They can also help prevent a Trojan Horse from communicating with its command and control server, limiting the damage it can cause.

Impact of trojan horses

Trojan Horses can have a significant impact on individuals and organizations. For individuals, a Trojan Horse can lead to identity theft, financial loss, and damage to personal files. For organizations, a Trojan Horse can lead to data breaches, financial loss, damage to reputation, and even legal consequences.

Furthermore, the impact of a Trojan Horse can extend beyond the infected system. For example, a DDoS Trojan can use the infected system as part of a botnet to carry out attacks on other systems, causing widespread disruption.

Case studies

There have been many high-profile cases of Trojan Horse attacks. One of the most famous is the Stuxnet worm, a sophisticated Trojan that was used to sabotage Iran's nuclear program. Another notable example is the Zeus Trojan, which was used to steal banking information and carry out financial fraud on a massive scale.

These cases highlight the potential for Trojan Horses to cause significant harm and disruption, and underscore the importance of good cybersecurity practices and the use of reliable security software.


In conclusion, a Trojan Horse is a type of malware that disguises itself as a harmless or beneficial program to trick users into running it. Once installed, it can carry out a variety of malicious activities, from stealing personal information to damaging files or creating a backdoor for remote access.

Protecting against Trojan Horses requires a combination of good cybersecurity practices, such as being wary of downloading software or opening email attachments from unknown sources, and the use of reliable security software. With the increasing sophistication and prevalence of Trojan Horses, understanding this threat and how to defend against it is more important than ever.

This post has been updated on 17-11-2023 by Sofie Meyer.

Author Sofie Meyer

About the author

Sofie Meyer is a copywriter and phishing aficionado here at Moxso. She has a master´s degree in Danish and a great interest in cybercrime, which resulted in a master thesis project on phishing.

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