A Trojan horse is a malicious program that appears legitimate. The program claims to do one thing, such as be a game or a form to fill out to win a prize, but instead it does damage when it runs. Users are tricked into installing the software because they trust the sender. A common trick is to send an email to the user stating that the software will protect computers from viruses, worms, and spyware for free. The attached file is usually an exe or zip file.

Unlike viruses and worms, Trojan horses do not replicate themselves. Instead they delete files, destroy information, or open a backdoor into the computer so the attacker to gain access to steal information or damage the system.

To prevent Trojan horses, employees should be taught to never open or download a file that they did not request. Anti-virus software should be installed on every company computer and updated regularly. That software should scan every incoming and outgoing email and scan all files on the computer at least once a week. A firewall is also beneficial. Finally, make sure that all computers have the latest version of operating system and browsers.