A Trojan horse, or Trojan, is a type of malware that masquerades as a legitimate file or helpful program with the ultimate purpose of granting a hacker unauthorized access to a computer. Trojans do not attempt to inject themselves into other files like a computer virus. Trojan horses can make copies of themselves, steal information, or harm their host computer systems.Many trojans rely on drive-by downloads or install via online games or internet-driven applications in order to reach target computers. The term is derived from the Trojan Horse story in Greek mythology because Trojan horses employ a form of “social engineering,” presenting themselves as harmless, useful gifts, in order to persuade victims to install them on their computers.