What is a Trojan Horse's Trapdoor?

For a definition of Trojan Horse see the glossary or Attacks Utilizing a Trojan Horse.

A Trojan has two parts: server and client. The server part is a program or file that is installed on the prospective but unknowing victim's machine to infect it. The client part is on the attacker's system. This combination of software establishes a connection between the victim's machine and the attacker via the Internet.

Using a Trojan, an attacker can do almost the same actions remotely on the infected computer as if he were right there directly using it. The attacker can copy, view and delete information from the hard drive, run various applications, change configurations and settings, control the infected computer's hardware and much more. Moreover, it can all be done without the infected computer's user being aware that his computer is being accessed remotely. Often it requires a security expert to discover the situation. Typically Trojan horses are distributed over the Internet as small utility programs, screen-savers, and wallpaper for desktops. It just takes one of these for a cracker to access your system and vandalize it.

To avoid being infected with a Trojan, download and execute files only from trusted sources. You should also install and use: anti-virus programs, anti-Trojan software and a firewall.