What is a Trojan Horse Virus- By: Musarrat Sheikh

Description : Trojan horse is a term derived from the Trojan horse story in Greek mythology and it is software that appears to perform a desirable function for the user prior to run or install but instead steals information or harms the system. The purposes of this software are various one being Adware where the horse modifies the user's computer to display advertisements in undesirable places like the desktop or in uncontrollable pop ups. In some cases, it can be less notorious such as installing a toolbar on to the user's web browser without notice. This will thereby create revenue for the author of the Trojan despite it being against the terms of service of several internet-advertising networks such as Google Ad Sense. Trojan horse can also be used for security purposes and this is where it allows a hacker to remotely access to a target computer system.
Once a Trojan horse is installed in a target computer system a hacker can access to the computer remotely and perform several operations although they will be limited to the user's privileges on the target computer system as well as the design of the Trojan horse. There are several operations that a hacker can perform on a target computer and this will include:

Keystroke logging.
Crashing the computer.
Watching the user's screen.
Data theft like retrieving passwords or credit card information.
Modification and deletion of files.
Downloading and uploading files on the user's computer.
Installation of software, which will include third party malware.

Usage of the machine as part of a bonnet for example to perform automated spamming or distribute denial of service attacks among other operations.
Trojan horses in these cases will require a hacker's interaction to fulfill their purpose though the hacker need not be the individual responsible for distributing the Trojan horse. Some individual hackers scan computers on a network using a port scanner hoping to find one with a malicious Trojan horse installed where they can use it to control the target computer.

Currently Trojan horse is becoming more popular and this is because of the popularity of bonnets among hackers as well as the availability of advertising services that permit authors to violate their user's privacy. Trojan horse is more like a virus although the main difference is that Trojans spread themselves. Trojan horses disguise themselves as valuable and useful software available for download in several internet web sites where most people will be fooled and end up downloading it. The Trojan horse is typically separated in two parts a server and a client.

The client is cleverly disguised as significant software and also positioned in peer-to-peer file sharing networks or unauthorized download websites. Once the client excites on your computer the attacker who is the person, running the server gets a high level of control over your computer and this is what leads to destructive effects, which will mainly depend on the attacker's main purpose. Trojan horse spreads in a number of ways with most common way being through email attachments. Other will include via chat software's such as Skype and Yahoo Messenger or sending copies of the Trojan horse to people in the address book of an affected computer.

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Author Resource : This article touched the basics of the topic. I have 2 more resources related to the above. They are trojan horse and trojan horse virus. They are worth a read.