A program that appears to be non-threatening, but in fact has underlying malicious intent. This type of program is named after the famous Trojan Horse used by the Greek army to deceptively defeat Troy in Virgil's Aeneid.

In the story, the Greeks build a large wooden horse and claim that it is an offering to the Gods in return for s safe sea journey home. The Trojans brought the horse into the city without - not realizing that inside the wooden horse were hidden Greek soldiers. At night, the Greek soldiers emerged from hiding and opened the gates of Troy to the greater Greek army laying in wait - leading to the destruction of the city.

Malicious software disguises itself as a program that either naturally belongs or seems harmless. Many trojan horse programs open backdoors that cyber criminals can use to gain wider access to the target system - just like in the story.