What are trojan horses?
Trojan horses, otherwise known as trojans, are programs or applications that are
inadvertently opened by the user, who expects the file to be something else.
How do they get on your machine?
Trojans may come into your system as an email attachment, as a file via your
instant-messaging application, or even through file-sharing software.
The file may appear to be a photograph, a document, a sound file or even a game
of some sort. When you try to open the file, it often appears as though nothing
has happened. At this point, many users assume that the file “didn’t work” and
forget the whole thing.
In actual fact, you nay just have installed something quite nasty on your
system, and chances are that the Trojan will make sure it runs every time you
start your computer. Effectively you’ve just installed a backdoor into your
system. Not only do you not know that it’s there, but it can be easily and
discretely used without your knowledge or permission.
What can they do to your system?
There are many different trojans in circulation. At the time of writing this
text, the a-squared Anti-Malware software can detect over 3 million different trojans and
malware files, and this number will continue to rise in the future.
Some of them may be relatively “harmless” to your system, and may simply use
your computer as part of a co-ordinated Denial of Service attack on another
system. Some of them may open up the possibility of someone accessing your
machine, but may remain unused for years.
Other Trojans pose a far greater effect to your computer and your data. Some
may monitor your internet connection and grab your email addresses and access
passwords. Perhaps your email will be intercepted, perhaps credit card details
that you store or enter will be copied, or perhaps someone, somewhere, will
simply have a wander through your hard drive, seeing if there’s anything of
When they’ve finished, maybe they’ll leave the door open to others, maybe
they’ll come back at a later date, or maybe they’ll install something even
worse on your system, just for the hell of it.
Whatever the Trojan, and whatever the result of it being there, one thing is
for certain. You don’t want it there, and you need to protect your system from
Are you at risk?
Very few computer systems are 100% isolated from the rest of the world. If your
system has a floppy disk drive or a CD player, then it’s open to infection. If
you’re connected to the internet or an intranet, then it’s equally at risk.
If you install software, listen to music files, share data, receive email or
surf the web, you’re at risk.
An incredible fact is that even if your system really is well and truly
isolated, is never updated, with no additional software ever being installed,
with no connection to any outside line, the internet or a network of any kind,
you still may be at risk. How? Someone, somewhere had to install the operating
system and software onto your hard drive. As unlikely as it may be, it is still
possible that a Trojan is lurking somewhere on your system. The fact that it
has no means of doing anything isn’t the point… it may still be there.
What is the difference between a Trojan Horse and a Virus?
Despite the fact that viruses and trojans are frequently confused, they are
actually two very different types of unwanted pests.
A virus is a small program that “adds itself” onto an existing program, and
forms a modified file. Because of this it is difficult to detect, as no new or
suspicious files will initially appear on the infected system.
Once activated, a virus will usually spread itself onto other systems – either
across a network, via email, through the internet, or even via disks and CDs.
A virus can delete or alter files and data, and may sometimes go to great
lengths to remain undetected, and may often prove to be extremely difficult to
remove. They often remain quite invisible to the unwary user, and may even
disable anti-virus software to remain hidden and safe from detection and
A trojan on the other hand is a standalone program. It does not add itself onto
existing files, nor does it try to replicated and spread to other systems and
A trojan’s main purpose it to make it’s host system open to access, usually via
the internet. A trojan may allow an unwanted person to see the files and data
on the infected system. It may grant free access to the intruder, who may then
be able to freely explore, view, modify and delete data from the hard drive.
They may be able to intercept incoming and outgoing data, including emails,
keystrokes, passwords, sensitive information and so on.
For the Trojan to be effective, it needs to be run at every reboot of the
infected system. Because of this it may add entries to the registry or even the
Removing the trojan tends to be quite simple. You simply delete the offending
file, and remove all references to it. Detecting it, however, in the tens of
thousands of files on most hard drives, is an altogether more difficult
Click here for further details of the a-squared Anti-Malware software, or simply
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