A Trojan horse, or simply a “Trojan”, is an unwanted or harmful computer program (malware) that disguises itself as a useful application. The name comes from the Trojan horse in Greek mythology. In this story, Greek soldiers hid inside a wooden horse to enter the city of Troy unnoticed and conquer it.
Trojans work in a very similar way. Disguised as legitimate software or a file, after successful infiltration Trojans install unwanted software, such as keyloggers, spyware or ransomware. These programs then run independently on the computer, even if the Trojan program has been terminated or already deleted. Since a Trojan program is usually started manually by the user, it is given the same access rights as the logged-on user. Very often these are administrator rights, which allow the malware to work undetected, either independently or by remote control.
By definition, however, a Trojan horse is any program to which disguised functionality has been added that is unrelated to the actual program. Therefore, not every Trojan automatically causes direct damage.
Disguised with a false file name or a double extension, Trojans spread via e-mail attachments, data carriers, file-sharing networks or via specially prepared websites. Unlike viruses or worms, however, Trojans cannot spread themselves or automatically. This is why worms are often used to transport a Trojan in an attachment.
