A Trojan horse is an unauthorized program contained within a legitimate program. This unauthorized program performs functions unknown (and probably unwanted) by the user.
- It is a legitimate program that has been altered by the placement of unauthorized code within it; this code performs functions unknown (and probably unwanted) by the user.
 - Any program that appears to perform a desirable and necessary function but that (because of unauthorized code within it that is unknown to the user) performs functions unknown (and definitely unwanted) by the user.
 
Working of Trojans

- Attacker gets access to the trojaned system as the system goes online
 - By way of the access provided by the trojan attacker can stage attacks of different types.
 
Various Trojan Types
- Remote Access Trojans
 - Password Sending Trojans
 - Keyloggers
 - Destructive
 - Denial Of Service (DoS) Attack Trojans
 - Proxy/Wingate Trojans
 - FTP Trojans
 - Software Detection Killers
 
Modes of Transmission
- Attachments
 - Physical Access
 - Browser And E-mail Software Bugs
 - NetBIOS (File Sharing)
 - Fake Programs
 - Un-trusted Sites And Freeware Software
 
Backdoor Countermeasures
- Most commercial ant-virus products can automatically scan and detect backdoor programs before they can cause damage (Eg. before accessing a floppy, running exe or downloading mail)
 - An inexpensive tool called Cleaner (http://www.moosoft.com/cleanet.html) can identify and eradicate 1000 types of backdoor programs and trojans.
 - Educate your users not to install applications downloaded from the internet and e-mail attachments
 
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