In-depth

What is a Trojan?

The malicious malware lurks behind legitimate software to invade your computer

Trojan virus

It'll come as no surprise to hear that a Trojan virus takes its name from the legend of the siege of Troy, hiding itself inside legitimate-looking - or in some cases, completely legitimate and unharmful - software to infiltrate a network and cause harm.

And just like the 2,500-year-old legend, where an invading Greek army hid in a giant wooden horse to fool the opposition, a Trojan virus can be extremely damaging. It may not be able to take down an entire city (or at least that would most definitely be a worst-case scenario), but it can certainly attack a business' infrastructure from within and spread fast.

The malware camouflages itself as legitimate software to dodge any antivirus or other security system set up to resist attacks and then reveals itself when it's too late and the malicious code has spread or completed its mission.

Trojan viruses can stay dormant for a while, spying on the user's computer without any evidence of malice at all. However, it's already plotting its next move at this point, which could be modifying data on a computer, copying information and sending back to its developer or even worse, blocking access to data or draining a machine of its compute resource.

Types of Trojan

There's a wide range of Trojan types out there, each named according to the methods they deploy to attack a system.

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As the name suggests, Backdoor Trojans are designed to grant an external user full control over a system by creating a breach in a system - known as a 'backdoor'. This can then be exploited to repeatedly gain access to the system to steal or spy on its contents. The longer this door is open, the more damage a hacker can do.

Download Trojans get their name from being able to download other malware types once it has worked its way onto a system. Although a range of malicious tools can be installed, these Trojans often turn to keyloggers and cryptocurrency miners to secretly harvest data, often ending the attack with a ransomware infection.

Banking Trojans are so-called for their penchant for financial gain, typically lying in wait on a system until a user attempts to access their online banking service. The Trojan triggers by sending the user instead to a fake website, designed to mimic a legitimate bank, where it attempts to trick the user into entering their details.

Banking Trojans have been highly successful in the past, with notable examples such as Zeus, Dridex and Kronos. With today's heightened security and proactive attempts to clamp down on this style of attack, banking Trojans aren't as prevalent as they once were.

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.

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

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