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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 694
SEC. 9.9
9.9.1 Trojan Horses
Writing malware is one thing. You can do it in your bedroom.
Getting millions
of people to install it on their computers is quite something else. How would our
malware writer, Mal, go about this?
A very common practice is to write some gen-
uinely useful program and embed the malware inside of it. Games, music players,
‘‘special’’ porno viewers, and anything with splashy graphics are likely candidates.
People will then voluntarily download and install the application.
As a free bonus,
they get the malware installed, too. This approach is called a
Trojan horse
after the wooden horse full of Greek soldiers described in Homer’s
. In the
computer security world, it has come to mean any malware hidden in software or a
Web page that people voluntarily download.
When the free program is started, it calls a function that writes the malware to
disk as an executable program and starts it. The malware can then do whatever
damage it was designed for, such as deleting, modifying, or encrypting files. It can
also search for credit card numbers, passwords, and other useful data and send
them back to Mal over the Internet. More likely, it attaches itself to some IP port
and waits there for directions, making the machine a zombie, ready to send spam
or do whatever its remote master wishes. Usually, the malware will also invoke the
commands necessary to make sure the malware is restarted whenever the machine
is rebooted.
All operating systems have a way to do this.
The beauty of the Trojan horse attack is that it does not require the author of
the Trojan horse to break into the victim’s computer. The victim does all the work.
There are also other ways to trick the victim into executing the Trojan horse
program. For example, many UNIX users have an environment variable,
which controls which directories are searched for a command.
It can be viewed by
typing the following command to the shell:
echo $PATH
A potential setting for the user
on a particular system might consist of the fol-
lowing directories:
Other users are likely to have a different search path. When the user types
to the shell, the shell first checks to see if there is a program at the location
If there is, it is executed. If it is not there, the shell tries
, and so on, trying all 10 directories in
turn before giving up. Suppose that just one of these directories was left unprotect-
ed and a cracker put a program there.
If this is the first occurrence of the program
in the list, it will be executed and the Trojan horse will run.

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