A backdoor Trojan allows someone to take control of another user’s computer via the internet without their permission.

A Backdoor Trojan may pose as legitimate software to fool users into running it. Alternatively – as is now increasingly common – users may allow Trojans onto their computer by following a link in a spam mail or visiting a malicious web page.

Once a Trojan runs, it adds itself to the computer’s start-ups routine. It can then monitor the computer until the user is connected to the internet. When the computer goes online, the person who sent the Trojan can perform many actions – for example, run programs on the infected computer, access personal file, modify and upload files, track the user’s keystrokes, or send out spam e-mail.

Well known backdoor Trojans include Zapchast, Subseven, BackOrifice and, more recently PcClient.

To avoid backdoor Trojans, you should keep your computers up to date with the latest patches (to close down the vulnerabilities in the operating system), and run anti-spam and anti-virus software. You should also run a firewall, which can prevent Trojans from accessing the internet to make contact with a hacker.