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Trojan Horse Definition & Meaning

A trojan horse, or trojan, is a form of malware that disguises itself as a harmless file or application to mislead users of its true objective. The trojan will be released on the user’s device with a click or download of the seemingly innocent program. This allows the malicious code to perform whatever task the attacker intended. Unlike viruses and worms, trojans do not reproduce or self-replicate.

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.

Types of trojan horses

  • Backdoor allows attackers full access and control of the computer it infects.
  • Banker steals banking account information.
  • Data Sending sends sensitive data from a device to the attacker by searching the device or keylogging.
  • Distributed denial of service (DDoS) uses multiple devices infected by trojans to overwhelm and flood a network which leads to a denial of service.
  • Destructive trojans destroy and delete files.
  • Downloader trojans download and install malicious software and programs.
  • Game-thief steals online gamer’s account information.
  • Mailfinder steals email addresses from your mail history.
  • 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.
  • Rootkit hides the malware on your device to extend the time that the program can run.
  • SMS infects a mobile device and can send and intercept text messages.

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.

Signs of a trojan horse

  • 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









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