trojan horse

a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst

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"

a large hollow wooden figure of a horse (filled with Greek soldiers) left by the Greeks outside Troy during the Trojan War