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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
Page 708
SEC. 9.9
background. Neither of these are considered spyware. If Potter Stewart were alive,
he would probably say: ‘‘I can’t define spyware, but I know it when I see it.’’†
Others have tried harder to define it (spyware, not pornography). Barwinski et
al. (2006) have said it has four characteristics. First, it hides, so the victim cannot
find it easily.
Second, it collects data about the user (Websites visited, passwords,
even credit card numbers).
Third, it communicates the collected information back
to its distant master. And fourth, it tries to survive determined attempts to remove
it. Additionally, some spyware changes settings and performs other malicious and
annoying activities as described below.
Barwinsky et al. divided the spyware into three broad categories. The first is
marketing: the spyware simply collects information and sends it back to the master,
usually to better target advertising to specific machines. The second category is
surveillance, where companies intentionally put spyware on employee machines to
keep track of what they are doing and which Websites they are visiting. The third
gets close to classical malware, where the infected machine becomes part of a
zombie army waiting for its master to give it marching orders.
They ran an experiment to see what kinds of Websites contain spyware by vis-
iting 5000 Websites. They observed that the major purveyors of spyware are Web-
sites relating to adult entertainment, warez, online travel, and real estate.
A much larger study was done at the University of Washington (Moshchuk et
al., 2006).
In the UW study, some 18 million URLs were inspected and almost 6%
were found to contain spyware. Thus it is not surprising that in a study by
AOL/NCSA that they cite, 80% of the home computers inspected were infested by
spyware, with an average of 93 pieces of spyware per computer. The UW study
found that the adult, celebrity, and wallpaper sites had the largest infection rates,
but they did not examine travel and real estate.
How Spyware Spreads
The obvious next question is: ‘‘How does a computer get infected with spy-
ware?’’ One way is the same as with any malware: via a Trojan horse.
A consid-
erable amount of free software contains spyware, with the author of the software
making money from the spyware. Peer-to-peer file-sharing software (e.g., Kazaa)
is rampant with spyware. Also, many Websites display banner ads that direct
surfers to spyware-infested Web pages.
The other major infection route is often called the
drive-by download
. It is
possible to pick up spyware (in fact, any malware) just by visiting an infected Web
page. There are three variants of the infection technology.
First, the Web page may
redirect the browser to an executable (
) file. When the browser sees the file, it
pops up a dialog box asking the user if he wants to run or save the program. Since
legitimate downloads use the same mechanism, most users just click on RUN,
† Stewart was a justice on the U.S. Supreme Court who once wrote an opinion on a pornography case
in which he admitted to being unable to define pornography but added: ‘‘but I know it when I see it.’’

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