Trojan horse (computing)

In computing, a trojan horse (or simply trojan; often capitalized, but see below) is a kind of malware that misleads users as to its true intent by disguising itself as a normal program.

Trojans are generally spread by some form of social engineering. For example, a user may be duped into executing an email attachment disguised to appear innocuous (e.g., a routine form to be filled in), or into clicking on a fake advertisement on the Internet. Although their payload can be anything, many modern forms act as a backdoor, contacting a controller who can then have unauthorized access to the affected device. Ransomware attacks are often carried out using a trojan.

Unlike computer viruses and worms, trojans generally do not attempt to inject themselves into other files or otherwise propagate themselves.

The term is derived from the ancient Greek story of the deceptive Trojan Horse that led to the fall of the city of Troy.

It is unclear where and when the computing concept, and this term for it, originated; but by 1971 the first Unix manual assumed its readers knew both.

Another early reference is in a US Air Force report in 1974 on the analysis of vulnerability in the Multics computer systems.

The term "Trojan horse" was popularized by Ken Thompson in his 1983 Turing Award acceptance lecture "Reflections on Trusting Trust", subtitled: "To what extent should one trust a statement that a program is free of Trojan horses? Perhaps it is more important to trust the people who wrote the software." He mentioned that he knew about the possible existence of trojans from a report on the security of Multics.

The computer term "Trojan horse" is derived from the legendary Trojan Horse of the ancient city of Troy. For this reason "Trojan" is often capitalized, especially in older sources. However, many modern style guides and dictionaries suggest a lower-case "trojan" for this technical use.