trojan horse

(noun) a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst. Synonyms: fifth column, Trojan horse.

(noun) a program that appears desirable but actually contains something harmful. Synonyms: trojan, trojan horse. Examples: "The contents of a trojan can be a virus or a worm." "When he downloaded the free game it turned out to be a trojan horse."

(noun) a large hollow wooden figure of a horse (filled with Greek soldiers) left by the Greeks outside Troy during the Trojan War. Synonyms: Trojan Horse, Wooden Horse.