In computing, a Trojan horse is any malware that misleads users of its true intent. The term is derived from the Ancient Greek story of the deceptive Trojan Horse that led to the fall of the city of Troy. Trojans generally spread by some form of social engineering; for example, where a user is duped into executing an email attachment disguised to appear innocuous (e.g., a routine form to be filled in), or by clicking on some fake advertisement on social media or anywhere else. 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 computer. 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. Use of the term It's not clear where or when the concept, and this term for it, was first used, 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. It was made popular 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. Behavior Once installed, Trojans may perform a range of malicious actions. Many tend to contact one or more Command and Control (C2) servers across the Internet and await instruction. Since individual Trojans typically use a specific set of ports for this communication, it can be relatively simple to detect them. Moreover, other malware could potentially "take over" the Trojan, using it as a proxy for malicious action. In German-speaking countries, spyware used or made by the government is sometimes called govware. Govware is typically a Trojan software used to intercept communications from the target computer. Some countries like Switzerland and Germany have a legal framework governing the use of such software. Examples of govware Trojans include the Swiss MiniPanzer and MegaPanzer and the German "state Trojan" nicknamed R2D2. German govware works by exploiting security gaps unknown to the general