Trojan horse

A destructive program that masquerades as a benign application.
Unlike viruses, Trojan horses do not replicate themselves but
they can be just as destructive. One of the most insidious types
of Trojan horse is a program that claims to rid your computer
of viruses but instead introduces viruses onto your computer.

The term comes from the a Greek story of the Trojan War,
in which the Greeks give a giant wooden horse to their foes,
the Trojans, ostensibly as a peace offering. But after the Trojans
drag the horse inside their city walls, Greek soldiers sneak out
of the horse's hollow belly and open the city gates, allowing their
compatriots to pour in and capture Troy.