Trojan horse (computing)

In computing, a Trojan horse[1] (or simply trojan)[2] is any malware which 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.[1][2][3][4][5]

Trojans are generally spread by some form of social engineering, for example where a user is duped into executing an email attachment disguised to appear not suspicious, (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 which can then have unauthorized access to the affected computer.[6] Trojans may allow an attacker to access users' personal information such as banking information, passwords, or personal identity. It can also delete a user's files or infect other devices connected to the network. Ransomware attacks are often carried out using a trojan.

Unlike computer viruses, worms, and rogue security software, trojans generally do not attempt to inject themselves into other files or otherwise propagate themselves.[7]

Origin of the concept

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:[8]

Also, one may not change the owner of a file with the set—user—ID bit on, otherwise one could create Trojan Horses able to misuse other’s files.

Another early reference is in a US Air Force report in 1974 on the analysis of vulnerability in the Multics computer systems.[9]

It was made popular by Ken Thompson in his 1983 Turing Award acceptance lecture "Reflections on Trusting Trust",[10] 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.[11][12]

Malicious uses

Trojan viruses, in this way, may require interaction with a malicious controller (not necessarily distributing the trojan) to fulfill their purpose. It is possible for those involved with trojans to scan computers on a network to locate any with a trojan installed, which the hacker can then control, creating a so called botnet.[13]

Some trojans take advantage of a security flaw in older versions of Internet Explorer and Google Chrome to use the host computer as an anonymizer proxy to effectively hide Internet usage, enabling the controller to use the Internet for illegal purposes while all potentially incriminating evidence indicates the infected computer or its IP address. The host's computer may or may not show the internet history of the sites viewed using the computer as a proxy. The first generation of anonymizer trojan horses tended to leave their tracks in the page view histories of the host computer. Later generations of the trojan tend to "cover" their tracks more efficiently. Several versions of Sub7 have been widely circulated in the US and Europe and became the most widely distributed examples of this type of trojan.[13]

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.[14][15] Examples of govware trojans include the Swiss MiniPanzer and MegaPanzer[16] and the German "state trojan" nicknamed R2D2.[14] German govware works by exploiting security gaps unknown to the general public and accessing smartphone data before it becomes encrypted via other applications.[17]

Due to the popularity of botnets among hackers and the availability of advertising services that permit authors to violate their users' privacy, trojans are becoming more common. According to a survey conducted by BitDefender from January to June 2009, "trojan-type malware is on the rise, accounting for 83% of the global malware detected in the world." Trojans have a relationship with worms, as they spread with the help given by worms and travel across the internet with them.[18] BitDefender has stated that approximately 15% of computers are members of a botnet, usually recruited by a trojan infection.[19]

Notable examples

Private and governmental

Publicly available

Detected by security researchers

Orthographic note

The term "trojan horse" in computing is derived from the legendary Trojan Horse; itself named after Troy. For this reason "Trojan" is often capitalized. However, while style guides and dictionaries differ, many suggest a lower case "trojan" for normal use.[28][29] That is the approach taken in this article - apart from when first introducing the word and its derivation.

See also


1.^ Upper case is intentional here, please see the "Orthographic note" section, and the Talk page
2.^ Lower case is intentional here, please see the "Orthographic note" section , and the Talk page


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  11. ^ Paul A. Karger; Roger R. Schell (2002), "Thirty Years Later: Lessons from the Multics Security Evaluation" (PDF), ACSAC: 119–126
  12. ^ Karger et Schell wrote that Thompson added this reference in a later version of his Turing conference: Ken Thompson (November 1989), "On Trusting Trust.", Unix Review, 7 (11): 70–74
  13. ^ a b Jamie Crapanzano (2003): "Deconstructing SubSeven, the Trojan Horse of Choice", SANS Institute, Retrieved on 2009-06-11
  14. ^ a b Basil Cupa, Trojan Horse Resurrected: On the Legality of the Use of Government Spyware (Govware), LISS 2013, pp. 419–428
  15. ^ "Häufig gestellte Fragen (Frequently Asked Questions)". Federal Department of Justice and Police. Archived from the original on May 6, 2013.
  16. ^ Dunn, John (August 27, 2009). "Swiss coder publicises government spy Trojan". TechWorld. Archived from the original on January 26, 2014. Retrieved January 10, 2021.
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  25. ^ "Shuanet, ShiftyBug and Shedun malware could auto-root your Android". November 5, 2015.
  26. ^ Times, Tech (November 9, 2015). "New Family of Android Malware Virtually Impossible To Remove: Say Hello To Shedun, Shuanet And ShiftyBug".
  27. ^ "Android adware can install itself even when users explicitly reject it". November 19, 2015.
  28. ^ "trojan". Collins Advanced Dictionary. Retrieved March 29, 2020.
  29. ^ "trojan horse". Microsoft Style Guide. Microsoft. Retrieved March 29, 2020.

External links