The word Trojan horse has been derived from Greek mythology in which Greeks built a huge wooden horse for their soldiers to hide and then placed that horse in front of Troy gates. Troy citizen thought its is gift to them for victory over Greeks and thus Greeks soldiers successfully bypassed the huge wall of Troy and destroyed it.
In world of computers and security Trojans are considered as malicious security breaking software or program, which cause considerable amount of damage to both software and hardware of the system. Trojans are written to steal information from other systems and to exercise control over victim PC. They can be used for destructive as well as spying purposes.
A computer Trojan horse use to enter victim's PC using a legitimate program and has all privileges that the user has. For example a Trojan can appear as a video or image file, even when you'll open it, you'll see video or image(how we will discus it later). A Trojan is generally wrapped into another program thus giving it hidden functionality. Once infected with Trojan an attacker can use victim's PC for his/her malicious activities like stealing credit cards, personal information, cause a Denial Of Service Attack and even for hacking another systems.
Basically Trojans Are Classified As Follows:
Remote Access Trojans(RATs):
Remote Access Trojans are today better known as Remote Administration Tool. They provide full access over victim's system to attackers and enable him/her to remotely access files, private conversations, transactional data etc.
Data Sending Trojans:
These Trojans are used for stealing information like passwords, credit card numbers, companies private and confidential data etc. They do this by installing keystroke loggers in system and send record to attacker via ftp or emails.
Destructive Trojans:
As name suggests these are written solely for destructive purposes and can destroy OS by deleting core files of system.
Denial Of Service DoS Attack Trojans:
They enable an attacker to launch a Distributed Denial Of Service (DDoS) attack against another victim.
Proxy Trojans:
These Trojans turn victim's PC into a proxy server for attacker, enabling him/her to do any malicious activity online with full anonymity.
FTP Trojans:
These allow attacker to connect victim as a FTP server due to which he can download all files in victim PC using port 21.
Security Disablers:
These are special kinda Trojans specially used to attack security measures in victim PC.
Now the real reality check, today most of the Trojans have all those features mentioned above. A Trojan can surely act as perfect backdoor and invisible and remotely installable and uninstallable rootkit.
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