Definition

Trojan horse

This definition is part of our Essential Guide: Secure Web gateways, from evaluation to sealed deal
Contributor(s): Michael Cobb

In computing, a Trojan horse is a program that appears harmless, but is, in fact, malicious. The term comes from Greek mythology about the Trojan War. According to legend, the Greeks built a large wooden horse that the people of Troy pulled into the city. During the night, soldiers who had been hiding inside the horse emerged, opened the city's gates to let their fellow soldiers in and then overran the city.

Attackers have long used Trojan horses as a way to trick end users into installing malware. Typically, the malicious programming is hidden within an innocent-looking email attachment or free program, such as a game. When the user downloads the Trojan horse, the malware that is hidden inside is also downloaded. Once inside the computing device, the malicious code can execute whatever task the attacker designed it to carry out.

A Trojan horse containing malware may also be referred to as simply a Trojan or a Trojan horse virus. Unlike a true virus, however, malware in a Trojan horse does not replicate itself, nor can it propagate without the end user's assistance. Because the user is often unaware that he has installed a Trojan horse, the computing device's security depends upon its antimalware software recognizing the malicious code, isolating it and removing it.

Unexpected changes to computer settings and unusual activity even when the computer should be idle are strong indications that a Trojan or other malware is residing on a computer. To avoid being infected by Trojan malware, users should keep their antivirus software up to date, never download files or programs from untrusted sources, and always scan new files with antivirus software before opening them.

This was last updated in April 2016

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How invasive should system and browser warnings be about the dangers of downloading certain files to avoid a Trojan virus from being installed?
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It's a fine line. It has to be invasive enough to forestall the trojans and their system infections. And it has to be unobtrusive enough to stay out of the way of the work.

Thing is, we've all be warned over and over again and yet far too many employees put themselves and their companies at risk with every download. Frankly, I'm for blaring klaxons and a screen covered with warning notices. Just before an automated shutdown of the offending system prevents even more harm. Followed by mandatory instructions and zero tolerance. 
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My computer I am typing on had the Trojan Horse Virus. For me it is next to impossible, however if your system isn't invasive...   

Goodbye computer.
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I have downloaded a trojan before using my wife's laptop. Then I installed ESET Antivirus and removed the trojan without deleting the files.
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