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 What is a Trojan? - Trojans Explained

When people think of trojans and trojan viruses they normally think of a big wooden horse full of soldiers. Computer trojans are based on the same idea; they appear to be harmless when often they harbour a secret malicious backdoor letting in hackers.

Trojan infections are increasing all the time, but many of the people who are infected with trojans do not fully understand what a trojan is, or the real security risk that trojans are.


Trojans are commonly called viruses, but there are very few trojans that are real viruses. It is safe to say that real computer viruses are very uncommon these days. A lot of what people call viruses are in fact malware, worms and trojans. A virus has the ability to replicate itself whereas a trojan does not. Trojans rely often on people being tricked into running the malicous code.

The main trojan types are as follows:-

Fileserving Trojan: These trojans create a file server, usually an ftp server on the remote victims computer allowing a hacker to upload or download files, this is commonly used to upload a powerful remote administration trojan. Because some of these file server trojans are small, (some are just 8 kbs) they are easily bound to other files making no significant size change. These are most commonly found in games and funny programs that people send around the internet to amuse each other not realising they are infecting themselves and there friends with trojans


Remote Administration Trojans: These include Subseven, netbus, back orifice etc. These types of trojans give the hacker more power over the victims computer then the victim may have originally had. They include such functions as the ability to steal all passwords cached or not (this is done using key logging technology), modify the victims registry, upload, download, execute (run) files, and various other things like turning on a web cam and spying on a victim.

Password Sending Trojans: These trojans have one purpose and that is to steal passwords from the victim's computer and send them back to the hacker. The most common way these trojans communicate with the hacker is by email. It is pretty scary to think that your computer is sending a hacker secret emails with all your passwords.


Keylogger Trojans: These trojans log everything the victim types and either sends the info to the hacker by way of email or stores the typed info in a secret file located on the victims computer which the hacker then downloads using the client part of the trojan. Some keyloggers will even store logged keys on a remote web or ftp server ready for the hacker to download at a later date.

DDoS (Distributed Denial of Service) Trojans: A hacker infects a large number of victims with a DDoS trojan, then using the client part of the trojan he or she can connect either to all of them at once or send commands to a drone (a master server) that then sends the commands out to all the victims. These commands are normally to attack a single website or persons pc. These types of trojans have been used recently to bring down big sites.

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