trojan-horse

(n) a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst fifth column,
(n) a program that appears desirable but actually contains something harmful
- 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
trojan,
(n) a large hollow wooden figure of a horse (filled with Greek soldiers) left by the Greeks outside Troy during the Trojan War Wooden Horse,

fifth column trojan Wooden Horse