a Trojan horse
1. Something that initially seems innocuous but is ultimately bad or malicious. A reference to the myth in which Ancient Greek soldiers hid inside a giant wooden horse in order to gain access to the city of Troy. That personable new hire turned out to be a Trojan horse—she stole our intellectual property and sold it to the competitor!
2. A computer program that appears to be useful or harmless but secretly installs malicious code or software onto the infected computer. We have malware on our computer because that game you downloaded turned out to be a Trojan horse.
a Trojan horse
COMMON If you describe someone or something as a Trojan horse, you mean that they seem good or useful, but are really there to help something be harmed or destroyed in the future. Proposals for a golf course are now seen as a Trojan Horse for hotel and conference centres. This small reduction in the basic tax rate was merely a Trojan horse for the far more drastic cutting of the top rate from 70 to 28 per cent. Note: This refers to an ancient Greek story. The city of Troy was under siege from the Greeks. The Greeks built a large hollow wooden horse and left it secretly as a gift for the Trojans, who took it into the city. However, Greek soldiers were hiding inside the horse, and they were able to cause the destruction of the city.
a Trojan horse1 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.