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Modern Operating Systems by Herbert Bos and Andrew...
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Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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triggers the real login program to start and display the prompt of Fig. 9-27(a). The
user assumes that she made a typing error and just logs in again. This time, how-
ever, it works. But in the meantime, Mal has acquired another (login name, pass-
word) pair.
By logging in at many computers and starting the login spoofer on all
of them, he can collect many passwords.
The only real way to prevent this is to have the login sequence start with a key
combination that user programs cannot catch.
Windows uses CTRL-ALT-DEL for
this purpose.
If a user sits down at a computer and starts out by first typing CTRL-
ALT-DEL, the current user is logged out and the system login program is started.
There is no way to bypass this mechanism.
In ancient times (say, before 2000), bored (but clever) teenagers would some-
times fill their idle hours by writing malicious software that they would then re-
lease into the world for the heck of it. This software, which included Trojan horses,
viruses, and worms and collectively called
, often quickly spread around
the world. As reports were published about how many millions of dollars of dam-
age the malware caused and how many people lost their valuable data as a result,
the authors would be very impressed with their programming skills.
To them it
was just a fun prank; they were not making any money off it, after all.
Those days are gone. Malware is now written on demand by well-organized
criminals who prefer not to see their work publicized in the newspapers. They are
in it entirely for the money. A large fraction of all malware is now designed to
spread over the Internet and infect victim machines in an extremely stealthy man-
ner. When a machine is infected, software is installed that reports the address of the
captured machine back to certain machines.
is also installed on the
machine that allows the criminals who sent out the malware to easily command the
machine to do what it is instructed to do.
A machine taken over in this fashion is
called a
, and a collection of them is called a
, a contraction of
‘‘robot network.’’
A criminal who controls a botnet can rent it out for various nefarious (and al-
ways commercial) purposes.
A common one is for sending out commercial spam.
If a major spam attack occurs and the police try to track down the origin, all they
see is that it is coming from thousands of machines all over the world. If they ap-
proach some of the owners of these machines, they will discover kids, small busi-
ness owners, housewives, grandmothers, and many other people, all of whom vig-
orously deny that they are mass spammers.
Using other people’s machines to do
the dirty work makes it hard to track down the criminals behind the operation.
Once installed, malware can also be used for other criminal purposes. Black-
mail is a possibility.
Imagine a piece of malware that encrypts all the files on the
victim’s hard disk, then displays the following message:

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