Trojan Horse or Virus?
by Joseph Lo aka Jolo
Part of the Trojan horse attacks
updated May 6, 2000
In May 2000, the "Love Bug" spread like wild fire and affected countless
computers around the world. The media described it with many technical
buzzwords. A CNN story
called it a "hybrid virus and worm", others have tossed around buzzwords like
Trojan horse, hacking, and cracking. Which is it?
If you're just a regular computer user, you don't really need to know these
differences, but we wanted to try to make this confusing subject as clear as
- A virus is a program that propagates itself by infecting other programs on
the same computer. Viruses can do serious damage, such as erasing your files
or your whole disk, or they may just do silly/annoying things like pop up a
window that says "Ha ha you are infected!" True viruses cannot spread to a new
computer without human assistance, such as if you trade files with a friend
and give him an infected file (such as on a floppy or by an email attachment).
- Like a virus, a worm is also a program that propagates itself. Unlike a
virus, however, a worm can spread itself automatically over the network from
one computer to the next. Worms are not clever or evil, they just take
advantage of automatic file sending and receiving features found on many
- This is a very general term, referring to programs that appear desirable,
but actually contain something harmful. The harmful contents could be
something simple, for example you may download what looks like a free game,
but when you run it, it erases every file in that directory. The trojan's
contents could also be a virus or worm, which then spread the damage. See our
help page for more information.
- Crackers are often mistakenly called "hackers". Crackers are the "bad
guys" who seek to "crack" or gain unauthorized access to computers, typically
to do malicious things e.g. to steal credit card information or crash the
computer. Crackers might do this by writing a virus, worm, or Trojan horse.
Alternatively, they may just exploit weaknesses in the computer's operating
system in order to gain entry. Many crackers will install a "backdoor" which
allows the cracker to "remote control" your computer over the internet, such
as to distribute child porn or perform a denial of service attack
against somebody else. Most crackers are just bored, anti-social kids who
aren't particularly smart and just take advantage of well-known, existing
exploits or the gullibility of the typical internet user.
- When used properly, this term refers to an elite breed of "good guys" who
are talented computer programmers. They enjoy solving challenging problems or
exploring the capabilities of computers. Like a carpenter wielding an axe to
make furniture, the hacker does good things with his skills. True hackers
subscribe to a code of ethics and look down upon the illegal and immoral
activity of crackers (defined above). When the press uses "hackers" to
describe virus authors or computer criminals who commit theft or vandalism, it
is not only incorrect but also insulting to true hackers.
Land of Confusion
OK so you think you've got those terms all straight in your head? Prepare to
be confused. :-)
Remember the "Love Bug"? Is it a virus, worm, or trojan?
Answer: all three! It's a trojan because it pretends to be a love letter when it
is really a harmful program. It's a virus because it infects all the image files
on your disk, turning them into new trojans. Finally, it's also a worm because
it propagates itself over the internet by hiding in trojans that it sends out
using your email address book, IRC client, etc.
Here's another one. Traditionally you use anti-virus programs to check
your computer for viruses and prevent their spread. The problem is, traditional
viruses don't really exist any more. Nowadays, lame crackers are busy
making trojans and worms, so that's what anti-virus programs try to tackle now.
The problem is, with everybody online these days, trojans and worms are fast to
spread and easy to modify, so anti-virus programs are useless in trying to
prevent them. If you're lucky, then can remove the infection after the fact,
assuming your disk isn't so messed up there is nothing left to disinfect.
Oh that's not all. Firewalls are network barriers designed to keep out
crackers. With the recent proliferation of trojans which install a
backdoor program, however, a whole new market has sprung up in "personal
firewalls" which are programs that run on your PC and can block communications
from some backdoor programs. Since file downloads are a normal part of your
internet experience, however, personal firewalls can't stop you from downloading
the trojan that installs that backdoor in the first place.
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