Trojan Horse (Computing)

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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.

1 Example
2 Types of Trojan horse payloads
3 Methods of deletion
4 How Trojans work
5 See also
- 5.1 Notable instances
6 References
7 External links

Example

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^[clarify]^{[dubious – discuss]}.

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).