In the context of computing and software, a Trojan horse, also known as a trojan, is a form of malware that appears to perform a desirable function but in fact performs undisclosed malicious functions that allow unauthorized access to the host machine.

As such, a computer worm or virus may also be classed as a Trojan horse if they display these characteristics.

The term is derived from the classical story of the Trojan Horse.

Exmaple: A program named "waterfalls.scr" serves as a simple example of a Trojan horse. The author claims it is a free waterfall screen saver.

When running, it instead unloads hidden programs, scripts, or any number of commands without the user's knowledge or consent.

Malicious Trojan horse programs are used to circumvent protection systems, in effect creating a vulnerable system to allow unauthorized access to the user's computer.

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