Trojan horse (computing)

A Trojan horse, or Trojan, in computing is a non-self-replicating type of malware program containing malicious code that, when executed, carries out actions determined by the nature of the Trojan, typically causing loss or theft of data, and possible system harm. The term is derived from the story of the wooden horse used to trick defenders of Troy into taking concealed warriors into their city in ancient Greece, because computer Trojans often employ a form of social engineering, presenting themselves as routine, useful, or interesting in order to persuade victims to install them on their computers.[1][2][3][4][5]
A Trojan often acts as a backdoor, contacting a controller which can then have unauthorized access to the affected computer.[6] The Trojan and backdoors are not themselves easily detectable, but if they carry out significant computing or communications activity may cause the computer to run noticeably slowly. Malicious programs are classified as Trojans if they do not attempt to inject themselves into other files (computer virus) or otherwise propagate themselves (worm).[7] A computer may host a Trojan via a malicious program a user is duped into executing (often an e-mail attachment disguised to be unsuspicious, e.g., a routine form to be filled in) or by drive-by download.

A Trojan may give a hacker remote access to a targeted computer system. Operations that could be performed by a hacker, or be caused unintentionally by program operation, on a targeted computer system include:

• Use of the machine as part of a botnet (e.g. to perform automated spamming or to distribute Denial-of-service attacks)
  Electronic money theft
  Data theft, including confidential files, sometimes for industrial espionage, and information with financial implications such as passwords and payment card information
  Modification or deletion of files
  Downloading or uploading of files for various purposes
  Downloading and installing software, including third-party malware and ransomware
  Keystroke logging
  Watching the user's screen
  Viewing the user's webcam
  Controlling the computer system remotely
  Encrypting files; a ransom payment may be demanded for decryption, as with the CryptoLocker ransomware
  Modifications of registry
  Using computer resources for Bitcoin mining.
  Linking computer to Botnet
  Using infected computer as proxy for illegal activities and attacks on other computers.
  More likely to be unintended or merely malicious, rather than criminal, consequences:
  Crashing the computer, e.g. with "blue screen of death" (BSOD)
  Data corruption
  Formatting disks, destroying all contents

Common Trojan horses:

• PC Optimizer Pro (unknown creator)
  Netbus (by Carl-Fredrik Neikter)
  Subseven or Sub7(by Mobman)
  Back Orifice (Sir Dystic)
  Beast
  Zeus
  Trojan.Agent
  The Blackhole exploit kit[12]
  Flashback Trojan (Trojan BackDoor.Flashback)
  ProRat
  ZeroAccess

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