Trojan horse (computing)
A Trojan horse, or Trojan, is a non-self-replicating type of malware which appears to perform a desirable function but instead drops a malicious payload, often including a backdoor allowing unauthorized access to the target's computer. These backdoors tend to be invisible to average users. Trojans do not attempt to inject themselves into other files like a computer virus. Trojan horses may steal information, or harm their host computer systems. Trojans may use drive-by downloads or install via online games or internet-driven applications in order to reach target computers. The term is derived from the Trojan Horse story in Greek mythology because Trojan horses employ a form of “social engineering,” presenting themselves as harmless, useful gifts, in order to persuade victims to install them on their computers.
Purpose and uses 
- Use of the machine as part of a botnet (e.g. to perform automated spamming or to distribute Denial-of-service attacks)
- Crashing the computer
- Blue screen of death
- Electronic money theft and disable all computer traffic of internet 
- Data theft (e.g. retrieving passwords or credit card information)
- Installation of software, including third-party malware and ransomware
- Downloading or uploading of files on the user's computer
- Modification or deletion of files
- Keystroke logging
- Watching the user's screen
- Viewing the user's webcam
- Controlling the computer system remotely
- Anonymizing remote third-party internet viewing
Trojan horses in this way may require interaction with a hacker to fulfill their purpose, though the hacker does not have to be the individual responsible for distributing the Trojan horse. It is possible for individual hackers to scan computers on a network using a port scanner in the hope of finding one with a malicious Trojan horse installed, which the hacker can then use to control the target computer.
A recent innovation in Trojan horse code takes 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. A hacker is able to view internet sites while the tracking cookies, internet history, and any IP logging are maintained on the host computer. 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. Newer 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 are the most widely distributed examples of this type of Trojan horse.
Current use 
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." This virus has a relationship with worms as it spreads with the help given by worms and travel across the internet with them.
Their main purpose is to make its host system open to access through the internet.
BitDefender also states that approximately 15% of computers are members of a botnet - usually an effect of a Trojan infection.
Common Trojan horses 
- Netbus (by Carl-Fredrik Neikter)
- Subseven or Sub7(by Mobman)
- Back Orifice (Sir Dystic)
- The Blackhole exploit kit
- Flashback Trojan (Trojan BackDoor.Flashback)
See also 
- Computer security
- Remote administration (Remote Access Tool trojans)
- Cyber spying
- Dancing pigs
- Exploit (computer security)
- Industrial espionage
- Principle of least privilege
- Privacy-invasive software
- Reverse connection
- Ransomware (malware)
- Rogue security software
- Social engineering (security)
- Timeline of computer viruses and worms
- "VIRUS-L/comp.virus Frequently Asked Questions (FAQ) v2.00 (Question B3: What is a Trojan Horse?)". 9 October 1995. Retrieved 2012-09-13.
- Landwehr, C. E; A. R Bull, J. P McDermott, W. S Choi (1993). "A taxonomy of computer program security flaws, with examples". DTIC Document. Retrieved 2012-04-05.
- "Trojan Horse Definition". Retrieved 2012-04-05.
- "Trojan horse". Webopedia. Retrieved 2012-04-05.
- "What is Trojan horse? - Definition from Whatis.com". Retrieved 2012-04-05.
- "Trojan Horse: [coined By MIT-hacker-turned-NSA-spook Dan Edwards] N.". Retrieved 2012-04-05.
- Jamie Crapanzano (2003): "Deconstructing SubSeven, the Trojan Horse of Choice", SANS Institute, Retrieved on 2009-06-11
- BitDefender.com Malware and Spam Survey
- Datta, Ganesh. "What are Trojans?". SecurAid.
- Burt, Jeffrey (2012-04-19). "HP: Fewer but More Dangerous Software Security Vulnerabilities". eWeek.com. Ziff Davis. Retrieved 2012-04-20. "[...] Web exploit kits continued to be popular in 2011. HP pointed to the Blackhole Exploit Kit, which officials said is used by most hackers and hit an infection rate of more than 80 percent in late November 2011."
- Carnegie Mellon University (1999): "CERT Advisory CA-1999-02 Trojan Horses", Retrieved on 2009-06-10.