What is a Trojan horse?

The term Trojan horse is applied to malware that masquerades as a legitimate program but is in reality a malicious application. It may simply pretend to be a useful program or it may actually contain a useful function as cover for a destructive one. Screen savers are often used as a carrier. Trojan horses do not replicate themselves as do viruses and worms. However, a Trojan horse can be part of the payload of a worm and can be spread to many machines as part of a worm infestation. Many Trojan horses have been sent out as email attachments.

One favorite use of Trojan horses is to allow a malicious hacker ( more properly called a "cracker") to use systems of unsuspecting owners for attacking other machines or as zombies. Another use is for relaying spam or pornography. Yet another use is to steal account passwords and then relay them back to someone for fraudulent use. Trojans can also be destructive and wipe out files or create other damage. Recently, phishing scams have been making use of Trojans.

Sometimes social engineering is used to induce people to click on a link. Here's one that enticed people to try to download some photos:

Osama Bin Ladin was found hanged by two CNN journalists early Wednesday evening. As evidence they took several photos, some of which I have included here. As yet, this information has not hit the headlines due to Bush wanting confirmation of his identity but the journalists have released some early photos over the internet.

Instead of photos what they got was a Trojan.