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    Trojan Horse virus definition

    What’s a Trojan horse virus?

    A trojan horse virus, or trojan, is a form of malware. It disguises itself as a harmless file or application to mislead users into installing it on their device. Clicking or downloading this seemingly innocent program allows the malicious code to perform whatever task the attacker intended.

    The name for this malware comes from the Greek legend of the Trojan War, in which the Greeks gift the Trojans a giant, wooden horse. After the horse was accepted into Troy’s city walls, Greek soldiers emerged from their hiding place within the horse and opened the city gates, which led to the capture of the city.

    It’s technically incorrect to use the term “Trojan virus”, because this type of attack cannot self-replicate like a true virus does. With that in mind, let’s properly unpack the concept of a Trojan Horse attack.

    How do Trojan attacks work?

    As explained, a Trojan attack cannot manifest by itself – it needs to be implanted into the target system by the end user, or victim.

    Because Trojan attacks need the target to install the code themselves, they frequently involve social engineering tactics. In other words, creating an environment in which the target will “open the door” to the malicious code without suspicion.

    Trojan infection methods

    This means Trojan attacks may start in a number of diffent ways.

    • an email attachment from a familiar looking sender
    • an innocent looking pop-up that needs to be clicked
    • an email with a link in the text

    In the case of a Trojan attack, all of these are potential vehicles for the malicious code.

    Types of trojan horses

    There are a variety of different types of Trojan attack that might be deployed on a target computer or network.


    Allows attackers full access and control of the computer it infects.


    Trojans download and install malicious software and programs.

    Distributed denial of service (DDoS)

    Otherwise known as a Botnet, this attack uses multiple devices infected by trojans to overwhelm and flood a network which leads to a denial of service.


    Steals online gamers’ account information.


    Steals email addresses from your mail history.


    Hides the malware on your device to extend the time that the program can run.


    Steals banking account information.

    Data Sending

    sends sensitive data from a device to the attacker by searching the device or keylogging.


    DestructiveTrojans destroy and delete files.


    Proxy Trojans use the infected computer as a proxy or zombie to conduct illegal activities undetected.


    Ransom Trojans steal data or damage your computer, and the attacker requires a ransom to release your data or repair the damage.


    Infects a mobile device and can send and intercept text messages.

    Famous Trojan virus examples

    Beyond targeting individuals for small gains, Trojan attacks have become a mainstay of larger scale cyber warfare. Some of the most famous attacks have involved large entities, like companies and even states. Here are some of the most famous Trojan attacks to date, and how they operated.

    Stuxnet attack

    The Stuxnet trojan was a specialised attack designed to disrupt Industrial Control Systems. Deployed in 2010, it successfully disrupted Iran’s nuclear enrichment facilities, causing the centrifuge to malfunction, undetected. Over a period of moths, the faulty equipment destroyed a significant proportion of Iran’s nucelar resources, costing billions of Dollars. The code is thought to have been physically injected into the facility’s software on a USB drive.

    ILOVEYOU attack

    Disguising itself in an email with the subject line ILOVEYOU, this 2000 attack was the original Trojan virus example. When recipient opened the mail, the same message was automatically sent to all of their contacts, overriding files on their devices.

    Signs of a trojan horse

    A Trojan horse virus can mie undetected on your device for a long period. When it does manifest, symptoms might include:

    • Desktop changes
    • Increase of spam or pop-ups
    • Poor device performance
    • Unfamiliar downloads, add-ons, or applications
    • Changes to display color, clarity, or orientation
    • Strange device behavior

    The best way to detect a Trojan is by using a specialized Trojan scanner or malware removal software.

    How to avoid trojan horses

    As you can see from the list above, trojans can steal sensitive information and cause a lot of serious damage. Fortunately, there are a few steps you can take to avoid this malware:

    • Don’t open an attachment or run a program found in an email from an unknown sender.
    • Only download software from sources you fully trust.
    • Avoid clicking on pop-ups that promise free software.
    • Keep your computer and its software up to date.
    • Install and regularly run an antivirus program on your computer.