Trojan horse (computing)
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In computing, Trojan horse, or Trojan, is any malicious computer program which is used to hack into a computer by misleading users of its true intent. The term is derived from the Ancient Greek story of the wooden horse that was used to help Greek troops invade the city of Troy by stealth.
Trojans are generally spread by some form of social engineering, for example where a user is duped into executing an e-mail attachment disguised to be unsuspicious, (e.g., a routine form to be filled in), or by drive-by download. Although their payload can be anything, many moderns forms act as a backdoor, contacting a controller which can then have unauthorized access to the affected computer.
Purpose and uses
If installed or run with elevated privileges a Trojan will generally have unlimited access. What it does with this power depends on the motives of the attacker.
- Crashing the computer or device.
- Modification or deletion of files.
- Data corruption.
- Formatting disks, destroying all contents.
- Spread malware across the network.
- Spy on user activities and access sensitive information.
Use of resources or identity
- Use of the machine as part of a botnet (e.g. to perform automated spamming or to distribute Denial-of-service attacks)
- Using computer resources for mining cryptocurrencies 
- Using the infected computer as proxy for illegal activities and/or attacks on other computers.
- Infecting other connected devices on the network.
Money theft, ransom
- Data theft, including for industrial espionage
- User passwords or payment card information
- User personally identifiable information
- Trade secrets
Spying, surveilance or stalking
- Keystroke logging
- Watching the user's screen
- Viewing the user's webcam
- Controlling the computer system remotely
Trojan horses in this way may require interaction with a malicious controller (not necessarily distributing the Trojan horse) to fulfill their purpose. It is possible for those involved with Trojans to scan computers on a network to locate any with a Trojan horse installed, which the hacker can then control. .
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 horse 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 horse.
In German-speaking countries, spyware used or made by the government is sometimes called govware. Govware is typically a trojan horse 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.
Due to the popularity of botnets among hackers and the availability of advertising services that permit authors to violate their users' privacy, Trojan horses 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-percent 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. BitDefender has stated that approximately 15% of computers are members of a botnet, usually recruited by a Trojan infection.
Private and governmental
- FinFisher – Lench IT solutions / Gamma International
- DaVinci / Galileo RCS – HT S.r.l. (hacking team)
- 0zapftis / r2d2 StaatsTrojaner – DigiTask
- TAO QUANTUM/FOXACID – NSA
- Magic Lantern – FBI
- WARRIOR PRIDE – GCHQ
- Netbus – 1998 (published)
- Sub7 – 1999 (published)
- Back Orifice – 1998 (published)
- Beast – 2002 (published)
- Bifrost Trojan – 2004 (published)
- DarkComet – 2008 (published)
- Blackhole exploit kit – 2012 (published)
- Gh0st RAT – 2009 (published)
- MegaPanzer BundesTrojaner – 2009 (published)
Detected by security researchers
- Clickbot.A – 2006 (discovered)
- Zeus – 2007 (discovered)
- Flashback Trojan – 2011 (discovered)
- ZeroAccess – 2011 (discovered)
- Koobface – 2008 (discovered)
- Vundo – 2009 (discovered)
- Meredrop – 2010 (discovered)
- Coreflood – 2010 (discovered)
- Tiny Banker Trojan – 2012 (discovered)
- Shedun Android malware – 2015 (discovered) 
- Computer security
- Remote administration
- Remote administration software
- Cyber spying
- Dancing pigs
- Exploit (computer security)
- Industrial espionage
- Principle of least privilege
- Privacy-invasive software
- Reverse connection
- Rogue security software
- Social engineering (security)
- Timeline of computer viruses and worms
- Command and control (malware)
- Zombie (computer science)
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