a Trojan horse

1 a person or device intended to undermine an enemy or bring about their downfall.

2 a program designed to breach the security of a computer system, especially by ostensibly functioning as part of a legitimate program, in order to erase, corrupt, or remove data.

☞ In Greek mythology, the Trojan horse was a huge hollow wooden statue of a horse in which Greek soldiers concealed themselves in order secretly to enter and capture the city of Troy, an action which brought the ten-year siege of the city to an end.