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A harmful application (or routine) that hides it malicious activities by masquerading as a more useful, harmless and desirable application.

In many respects a Trojan is like a virus, except that it does not provide a means to copy itself. A Trojan instead relies upon the fact that it masquerades as something useful that people will recommend or copy-on the application. Trojan's are often found in software that is freeware, although it must be stressed that most freeware is free of viruses and Trojans.

Anti-virus applications will typically scan for many known Trojans. Anti-virus writers tend to classify a Trojan as one of the following depending on what the Trojan does:

  • Trojan Downloader - downloads and installs new software on the computer.
  • Trojan Dropper - installs new software on the computer.
  • Trojan Proxy - turns the computer into a proxy server providing anonymous access to the internet for the Trojan writer.
  • Trojan PSW - password stealer, hunts the computer for stored usernames and passwords and forwards these to the Trojan writer.
  • Trojan Spy - any form of spyware application, such as keyloggers, password stealers etc.

Trojans get their name from the classical Greek story of the Trojan horse, used in the Trojan war. The Trojan horse was a large wooden horse left as a gift from the Greeks to the Trojans, the Greeks then sailed away (apparently admitting defeat). The Trojans moved the horse inside their city walls and then celebrated their apparent victory. Unfortunately for the Trojans the Trojan Horse contains a number of Greek soldiers who then killed the guards and let the remainder of the Greek army into the city who then sacked Troy.

A Trojan is also known as a Trojan horse.

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