A Trojan horse is a harmful software application that has been disguised as something useful, such as a game or work-related tool. Once they have infiltrated your computer’s defenses, some Trojans will turn into malevolent viruses and spyware programs. Others will simply sit there and wait for the user to do something unintended that opens the door to other malware.
Trojans are also called Trojan viruses, Trojan horses, or simply Trojans. They are named after the famous Trojan horse from ancient Greek mythology. That wooden horse was brought into Troy, a city on the coast of what is now Turkey, by a large group of soldiers working for the Greeks under their king, Agamemnon. They planned to use the horse to conquer the city and make it their own.
The Trojans thought that they had taken a harmless-looking wooden horse into their defenses. They put it out in front of the city’s walls along with a large quantity of gifts for Troy’s king, Priam. Inside the horse were enemy soldiers, who then proceeded to kill everyone and burn Troy to the ground. The Trojan horse was actually a trick to help the Greeks enter the city.
Essentially, a Trojan horse is a very small program that has been designed to look like something innocuous so that it can infiltrate computer systems and then do something harmful to them. The most common types of Trojans are designed to allow a remote attacker to gain access to your computer via connections such as email and websites, or even by simply tricking you into downloading them in an email message.
Once the Trojan has gained access, it can do a lot of damage. It can steal your personal information and use it for identity theft. It can read the information on your hard drive. It can even make changes to computer files to destroy or change them so that they cannot be used anymore. The program that allows this sort of access is frequently called a Trojan virus, although it is not technically a virus.
The Following are Three Popular Types of Trojans:
1. Browser Poisoning Trojans
Browser poisoning Trojans are a type of computer virus that infects web browsers and are typically spread through email messages that carry Visual Basic scripts (VB Scripts) embedded inside as attachments. The most typical look-and-feel of these types of viruses is a malicious banner advertisement (in the form of a RealMedia video file or QuickTime movie) on an unrelated Web page or in an email message; however, common variants also include Java applets and ActiveX controls embedded in barcode image files.
Victims can be infected by visiting Web sites that carry these malicious advertisements, or by opening email messages containing these advertisements as attachments. The infection occurs once the site or message are opened, and the browser is redirected to visit a Web site of the perpetrators’ choosing. These types of viruses typically install themselves as a browser helper object (BHO) in Internet Explorer or as an ActiveX control or Java applet in other browsers.
2. Email Trojans
One of the most popular email Trojans is called the Eudora Trojan. Email Trojans are a form of computer virus that infect a victim’s computer when the user opens an email message containing one or more malicious attachments (e.g., Word documents, Excel spreadsheets, or PowerPoint presentations) as well as a Trojan horse itself, which tricks the user into opening it by acting like an attachment.
A Trojan email message is typically crafted to be sent from an innocent-looking sender such as a colleague or friend. The message sends the user to a Web site that looks legitimate but actually contains the Trojan component. When displaying the email in Internet Explorer or Outlook, the victim might notice that it appears to come from an account unrelated to their own.
An Eudora-type Trojan is disguised as an email message by placing its malicious component in Windows’ Inbox folder in a specially named Microsoft Outlook subfolder. The name of this particular folder is composed of random characters, and the file extension of the malicious program is “.exe.” Once the user tries to open the folder or click on any of its contents, the software is installed.
3. Trojan Droppers
A Trojan dropper is a type of computer virus that installs other programs on a victim’s computer when it is run. Trojan droppers have recently been developed that hide as a Windows “Add/Remove Programs” utility. When the user installs a Trojan dropper, the program is nearly impossible to uninstall.