Trojan- A dangerous antivirus

Special thanks to wikipedia.

Hi Friends,
I call it as dangerous not beacause of its results but it cannot be detected by most antivirus programmes.

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.

Origin of the term
The word 'Trojan horse' is generally attributed to Daniel Edwards of the NSA. He is given credit for identifying the attack form in the 1972 report "Computer Security Technology Planning Study". The term derives from the Trojan War, as mentioned in Homer's Iliad and Odyssey and Virgil's Aeneid: worn out by the long siege, the attacking Greeks built a giant wooden horse, ostensibly a peace offering, and pretended to sail away, but in fact left soldiers hidden inside the statue. After the Trojans brought the horse inside the city walls, the soldiers emerged, (through an opening in the bottom of the horse) opened the gates to the Greek armies, and sacked the city of Troy.
A classic example originated from computer pioneer Ken Thompson in his 1983 ACM Turing Award lecture. Thompson noted that it is possible to add code to the UNIX "login" command that would accept either the intended encrypted password or a specific special password, allowing a back door into the system with the latter password. Furthermore, Thompson argued, the C compiler itself could be modified to automatically generate the rogue code, to make detecting the modification even harder. Because the compiler is itself a program generated from a compiler, the Trojan horse could also be automatically installed in a new compiler program, without any detectable modification to the source of the new compiler.

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
Down loader
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 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

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.

Disguises
Most varieties of Trojan horses are hidden on the computer without the user's awareness. Trojan horses sometimes use the Registry, adding entries that cause programs to run every time the computer boots up. Trojan horses may also work by combining with legitimate files on the computer. When the legitimate file is opened, the Trojan horse opens as well.

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 his or her victim. This is called a 'reverse-connect' trojan. Many trojans nowadays also bypass many personal firewall installed on the victims computer (eg. Poison-Ivy).