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 Trojan
is a program installed on computers usually without
the owners permission or knowledge and are used
to gain access by unauthorised people to the computer.
Trojans appear to be safe, but they often
hide malicious computer code that can spread a virus
or worm.
Trojans can sit harmlessly on a computer
until the hacker activates it and gains access.The
term comes from the a Greek story of the Trojan
War, in which the Greeks give a giant wooden horse
to their foes, the Trojans, ostensibly as a peace
offering. But after the Trojans drag the horse inside
their city walls, Greek soldiers sneak out of the
horse's hollow belly and open the city gates, allowing
their compatriots to pour in and capture Troy.
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Trojans typically consist
of two parts, a client part and a server part. When
a victim (unknowingly) runs a Trojan server on his
machine, the attacker then uses the client part
of that Trojan to connect to the server module and
start using the Trojan. The protocol usually used
for communications is TCP, but some Trojans' functions
use other protocols, such as UDP, as well. When
a Trojan server runs on a victim’s computer,
it (usually) tries to hide somewhere on the computer;
it then starts listening for incoming connections
from the attacker on one or more ports, and attempts
to modify the registry and/or use some other auto-starting
method.
It is necessary for the attacker to know the victim’s
IP address to connect to his/her machine. Many Trojans
include the ability to mail the victim’s IP
and/or message the attacker via ICQ or IRC. This
system is used when the victim has a dynamic IP,
that is, every time he connects to the Internet,
he is assigned a different IP (most dial-up users
have this). ADSL users have static IPs, meaning
that in this case, the infected IP is always known
to the attacker; this makes it considerably easier
for an attacker to connect to your machine.
Most Trojans use an auto-starting method
that allows them to restart and grant an attacker
access to your machine even when you shut down your
computer. Trojan writers are constantly on the hunt
for new auto-starting methods and other such tricks,
making it hard to keep up with their new discoveries
in this area. As a rule, attackers start by “joining”
the Trojan to some executable file that you use
very often, such as explorer.exe, and then proceed
to use known methods to modify system files or the
Windows Registry.
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