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A trojan is a program that is disguised as legitimate software but is designed to carry out some harmful actions on the infected computer.

Unlike viruses and worms, trojans don’t replicate but they can be just as destructive.

These days trojans are very common. Trojans are divided into a number different categories based on their function or type of damage.

Software that is designed to launch advertisements, frequently pop-up ads, on a user’s computer and/or to redirect search results to promotional Web sites. Adware programs are often built into freeware or shareware programs, where the adware creates an indirect ‘charge’ for using the free program. Sometimes a trojan can silently download an adware program from a Web site and install it onto a user’s machine.

Hacker tools, or Browser Hijackers, can also download an adware program by exploiting a web browser’s vulnerability. Browser Hijackers may tamper with the browser settings, redirect incorrect or incomplete URLs to unwanted Web sites, or change the default home page. They can also re-direct a user’s searches to “pay-to-view” (often pornographic) Web sites.

Typically, many adware programs do not leave any marks of their presence in the system: they are not listed on Start | Programs; they add no icons to the system tray; and they don’t show up on the task list. In addition, adware programs seldom provide an uninstallation procedure, and attempts at manually removing them frequently result in failure of the original carrier program.

Browser Helper Object, or BHO, is a dynamic link library (DLL) that runs whenever Internet Explorer is started. Generally, BHOs are included in installation of third-party programs where they are offered as enhancements of the browser functionality. For example, many Internet Explorer plug-ins are in essence BHOs.

BHOs can be installed silently or “legitimately” when a user fails to read the fine print included in the freeware program’s EULA (End User License Agreement).

Because BHOs are small programs, they can do anything that other programs can do. It’s not easy to detect the BHOs installed on the computer. This enables hackers and other malevolent users to employ the BHO functionality in their interests, for example, secretly install adware programs or gather various statistics on the user’s browsing trends.

Of all trojans, backdoor trojans pose the greatest danger to users’ PCs because they give their authors remote control over infected computers. They are downloaded, installed, and run silently, without the user’s consent or knowledge. Upon installation, backdoor trojans can be instructed to send, receive, execute and delete files, gather and transfer confidential data from the computer, log all activity on the computer, and perform other harmful activities.

A type of trojan. The primary purpose of downloaders is to install malicious code on a user’s computer. However, they can enable other malicious uses. For example, they can be used to continually download new versions of malicious code, adware, or “pornware.” They are also used frequently used to exploit the vulnerabilities of Internet Explorer.

Downloaders are typically written in script languages such as VBS or JavaScript.

Hacker tools are utilities designed to help hackers gain control of remote computers in order to use them as zombies (in DoS attacks, for example), download other malicious programs into those computers, or use them for other malicious purposes.