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Trojan

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.

Adware

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.

Backdoor

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.

Downloader

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.

DoS

A DoS (Denial of Service) attack is designed to disrupt or stop the normal running of a Web site, server, or other network resource. Hackers or malware writers resort to various ways to achieve this. A DoS attack can commonly result in a server being flooded with more network traffic than it is capable of processing. This hinders or prevents the server’s normal operation and sometimes causes its complete failure.

Unlike a DoS attack, a DDoS attack employs multiple PCs. The hacker or malware writer normally uses one infected computer - “master” - to centrally coordinate the attack across other, so-called “zombie”, computers. Typically, the malware writer gains control of both master and zombie computers by exploiting a weakness in an application or the operating system on those computers, in order to install a trojan or other malicious code.