trojan horse

Trojan horse (computing)

This article refers to a form of malware in computing terminology. For other meanings, see Trojan Horse (disambiguation)

In the context of computing and software, a Trojan horse, also known as a trojan, is malware that appears to perform a desirable function but in fact performs undisclosed malicious functions. Therefore, a computer worm or virus may be a Trojan horse. The term is derived from the classical story of the Trojan Horse.


A program named "waterfalls.scr" serves as a simple example of a trojan horse. The author claims it is a free waterfall screensaver. When run, it instead unloads hidden programs, commands, scripts, or any number of commands without the user's knowledge or consent. Malicious Trojan Horse programs are used to circumvent protection systems in effect creating a vulnerable system to allow unauthorized access to the user's computer. Non-malicious Trojan Horse programs are used for managing and forensics.

Types of Trojan horse payloads

Trojan horse payloads are almost always designed to cause harm, but can also be harmless. They are classified based on how they breach and damage systems. The six main types of Trojan horse payloads are:

  • Remote Access
  • Data Destruction
  • Downloader
  • Server Trojan(Proxy, FTP , IRC, Email, HTTP/HTTPS, etc.)
  • Security software disabler
  • Denial-of-service attack (DoS)

Some examples of damage are:

  • Erasing or overwriting data on a computer
  • Re-installing itself after being disabled
  • Encrypting files in a cryptoviral extortion attack
  • Corrupting files in a subtle way
  • Upload and download of files
  • Copying fake links, which lead to false websites, chats, or other account based websites, showing any local account name on the computer falsely engaging in untrue context
  • Falsifying records of downloading software, movies, or games from websites never visited by the victim.
  • Allowing remote access to the victim's computer. This is called a RAT (remote access trojan)
  • Spreading other malware, such as viruses (this type of trojan horse is called a 'dropper' or 'vector')
  • Setting up networks of zombie computers in order to launch DDoS attacks or send spam.
  • Spying on the user of a computer and covertly reporting data like browsing habits to other people (see the article on spyware)
  • Making screenshots
  • Logging keystrokes to steal information such as passwords and credit card numbers
  • Phishing for bank or other account details, which can be used for criminal activities
  • Installing a backdoor on a computer system
  • Opening and closing CD-ROM tray
  • Playing sounds, videos or displaying images
  • Calling using the modem to expensive numbers, thus causing massive phone bills
  • Harvesting e-mail addresses and using them for spam
  • Restarting the computer whenever the infected program is started
  • Deactivating or interfering with anti-virus and firewall programs
  • Deactivating or interfering with other competing forms of malware
  • Randomly shutting off the computer
  • Installing a virus
  • slowing down your computer
  • displaying pornographic sites

Methods of deletion

Since Trojan horses have a variety of forms, there is no single method to delete them. The simplest responses involve clearing the temporary internet files file and deleting it manually. Normally, anti-virus software is able to detect and remove the trojan automatically. If the antivirus cannot find it, booting the computer from alternate media(cd) may allow an antivirus program to find a trojan and delete it. Updated anti-spyware programs are also efficient against this threat.

How Trojans work

Trojans usually consist of two parts, a Client and a Server. The server is run on the victim's machine and listens for connections from a Client used by the attacker.

When the server is run on a machine it will listen on a specific port or multiple ports for connections from a Client. In order for an attacker to connect to the server they must have the IP Address of the computer where the server is being run. Some trojans have the IP Address of the computer they are running on sent to the attacker via email or another form of communication.

Once a connection is made to the server, the client can then send commands to the server; the server will then execute these commands on the victim's machine.

Today, with NAT infrastructure being common, most computers cannot be reached by their external ip address. Therefore many trojans now connect to the computer of the attacker, which has been set up to take the connections, instead of the attacker connecting to the victim. This is called a 'reverse-connect' trojan. Many trojans nowadays also bypass many personal firewall installed on the victims computer (eg. Poison-Ivy).

See also

Notable instances


External links

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