In-depth

What is a Trojan?

The malicious malware lurks behind legitimate software to invade your computer

Toy horse on a digital screen to symbolise the attack of the Trojan virus

Unfortunately, not everything we download or open is safe. What may look like a legitimate app or a safe document can occasionally be malware in disguise and the reason for a number of problems your device may suddenly suffer. 

Malware that is used in this way is called a Trojan - named after the wooden horse used to infiltrate the city of Troy - and is capable of causing lots of damage. 

Much like its namesake, the Greek soldiers within hid for long enough that their attack would cause the most damage. Trojan malware will sneak onto your machine and lay low and gather the information it needs to perform a number of malicious functions. While it remains undetected it will send info back to its creator, block access to data and even drain resources from its victim's machine. Trojan's have been used to steal financial information, in DDoS attacks and also as vehicles for other types of virus, such as ransomware. 

Trojans are widely available and relatively inexpensive, which is partly why they're so popular. A recent NCA investigation in 2019 found that remote access Trojans (RATs) were available for as little as $25 (£19). 

Beyond price and availability, Trojans are also considered some of the most effective tools for hackers as by the time most victims realise one is on their machine it is often too late. 

Types of Trojan

What is important to remember is that the term “Trojan” is actually just an umbrella term for a wide variety of malware types, from RATs to cryptocurrency miners. In fact, Trojans are usually named after the way they behave once they gain access to a system.

Backdoor Trojans, sometimes referred to as remote access Trojans (RATs), are built with the intention to allow cyber criminals to grasp full control over a system. They achieve this by creating a so-called backdoor that lets them come and go as they please for as long as the Trojan goes undetected, and can be used for an array of illegal activities, from spying on users to implementing larger cyber attacks.

Download Trojans, as their name suggests, are capable of downloading other malicious programmes once they gain access to a system. The most common tools are keyloggers, which harvest any usernames and passwords entered into the system, or cryptocurrency miners, which take advantage of a system’s processing power in order to subtly mine for Bitcoin as well as other digital tokens.

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Banking Trojans, otherwise known as 'Trojan bankers', focus primarily on financial gain. They are able to conceal themselves within a system, waiting for the moment when the user decides to access a financial service such as an online bank account. They then intercept this traffic and redirect their victim to a fraudulent website which usually contains data capture forms used to steal the victim’s information.

Banking Trojans have enjoyed considerable success in the past, with some famous examples including Zeus, Dridex, and Kronos. However, with today's heightened security measures as well as proactive efforts to prevent this style of attacks, banking Trojans aren't as common as they used to be.

Hands of man holding a smartphone and using a laptop computer to make an online purchase

How to protect against Trojans

While Trojans can cause significant damage if loaded on someone's system, there are ways to prevent malware from causing problems.

Simple steps such as avoiding unsafe websites and keeping accounts safe with secure passwords and firewalls can help prevent malware attacks. Updating a device's operating system as soon as possible will also help prevent Trojans from causing damage as malware tends to exploit the problems in outdated software.

It's also advisable to back up your files regularly, as if a Trojan infects your computer, this will help you to easily restore your data.

However, perhaps the most effective way of preventing this kind of malware attack is by installing anti-malware software on devices and running diagnostic scans with this software periodically.

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