What is a Trojan Horse?

A Trojan Horse is a piece of software that presents itself as being one thing, while in reality is another. In the 1980's they were common place on bulletin boards (BBS's) - posted as pranks by the cruelest of human beings - junior high students. A file might be described as a "Great new graphic file viewer," while in reality it erased your hard drive.

Today, most Trojans of this nature are distributed via file sharing networks. They may pose as executables claiming to be able to bypass software registration, or perform other acts of high desirability, but questionable legality.

Modern Trojans are more likely to install "backdoors" on your computer than erase your data. Once the security exploit is deployed, it's possible for hackers to manipulate your system remotely. A large network of compromised systems may use the exploit to coerce your computer into joining them. These networks are called botnets. They are frequently used by spammers to send unwanted messages by the millions without having to pay for computing or bandwidth expenses. Due to their highly diverse network topology, botnet spam is effective at defeating systems for blocking unwanted email.

Adware, spyware and malware are closely associated with Trojan Horses. That file-trading software or file compression application you downloaded does do what it claims to. However, it may install another piece of software in addition. At very least the software may display advertisements. Spyware sends information to third-parties who may use it for various purposes. Less malicious spyware may monitor aggregate web viewing trends. More malicious spyware may send contact or even financial data to third-parties without your permission.

In rare cases, adware is fairly legitimate. As long as the user is fully informed, there's nothing wrong with exchanging free use of software for viewing some advertisements. More commonly, the user isn't informed of the source of the ads and cannot readily uninstall the application once its been deployed.

Many users with serious pop-up advertisement problems have adware installed, but are not aware of it. Ad-aware can detect and remove most common spyware and adware.

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