Trojan (generic description)  
 
A trojan is usually a standalone program that performs destructive or other malicious or annoying actions when it's run (direct action trojan), after some period of time (time bomb) or in certain conditions (condition-triggered trojan).  
 
Destructive actions can vary from erasing or modifying the contents of files or sectors on a hard or floppy drive to a complete destruction of data on an entire media. Some trojans can destroy Flash Bios of certain types of motherboards and that will make an affected computer unbootable. Certain trojans can disable keyboard or mouse or only specific keys on a keyboard. Other trojans can disable certain features of an operating system. A few trojans perform annoying actions like changing position of icons on a desktop or showing video effects that makes a system hardly usable.  
 
Backdoor is a hacker's remote access trojan. Usually a backdoor is a standalone file that installs itself to system and then remains active there listening to specific network ports for specific commands. A typical backdoor consists of 2 parts - client and server. Some backdoor packages have configuration utilities that allow a hacker to configure server parts to their needs. A few backdoors have special scanner utilities to locate victim computers where server parts are installed.  
 
There also exist IRC backdoors. These backdoors are controlled via bots that they create in specific channels on selected IRC channels. These channels are usually invite-only, so they can be only accessed by hackers who use these backdoors.  
 
A server part of a typical backdoor is usually installed on a computer which is going to be accessed. Hackers use different tricks to infect users with server parts of backdoors - they send them in trojan dropper packages, give fancy names to server files and send them in e-mails. Some worms and viruses drop backdoors to infected systems.  
 
When a typical backdoor is run, it copies its file to Windows or Windows System folder and creates a Registry key to start that file during every Windows session. Also some backdoors modify WIN.INI and SYSTEM.INI files or copy themselves to startup folders for different users. After installation some backdoors can show fake error messages. Modern backdoors usually send a notification to specific e-mail, ICQ or MSN account when they are activated. They report infected computer's IP address and some other info. After a backdoor is installed, it starts to listen to certain network ports for specific commands coming from a client part.  
 
A client part of a backdoor is used to control a server part that is installed on a victim's computer. Client parts usually have a well-designed GUI (Graphical User Interface) to make communications with servers easy.  
 
The most advanced backdoors allow a hacker to get full control over an infected system. They include such features as sending and receiving files, browsing through victim's hard and network drives, getting system information, receiving a screenshot from a victim's computer, communicating with an infected user, change date/time and settings of operating system, play tricks (like open/close CD-ROM tray) and so on. Some backdoors even allow a hacker to listen and see what happens at a remote computer it it's equipped with a microphone and a webcam. Simple backdoors only allow upload, download and run files on victim's computers.  
 
An IRC backdoor is usually a standalone file that copies its file to Windows or Windows System folder and creates a Registry key to start that file during every Windows session. Also some IRC backdoors modify WIN.INI and SYSTEM.INI files or copy themselves to startup folders for different users. Some IRC backdoors replace INI scripts of an IRC client (mostly mIRC).  
 
When an IRC backdoor is run, it established connection to an IRC server or waits until a user connects to IRC (mIRC script-based backdoor). A backdoor then creates a bot in a specific channel on a specific IRC server. An IRC bot acts as a backdoor server interface. An IRC client in this case acts as a backdoor client. A hacker can give commands to an IRC bot using IRC interface.  
 
Most of advanced IRC backdoors allow to get a limited access to an infected system and to modify, upload, download and run files. Some IRC backdoors have additional functionalities that allow a hacker to perform malicious actions in IRC channels and in some cases can allow an attacker to completely take over an IRC channel.  
 
[Description: F-Secure Anti-Virus Research Team; F-Secure Corp.; July 14th, 2003]

Connected via WANADOO
 
Your computer transmit and contains many personal information and has been tracked.
Your IP address known. This means that using special software anyone (FBI, Police, hackers e.t.c.) can connect to your Windows OS and retreive any pictures, any files, history or other personal data from your computer.
This information can be used as evidence for any investigation or can be stolen.
 
Computer IP address:  
Home/office address: ON FILE. WANADOO have records  
You're using: Mozilla/4.0 (compatible; MSIE 6.0; Windows 98; Wanadoo 5.6)  
Risk status: HIGH RISK - your computer allows unsecure connections  
 
Your computer is full of evidence and unwanted tracks. You need professional help now!
Clean computer tracks and detect SPYWARE installed on your computer now!