User Guide

User’s Guide xv
Preface
Someof thesestudentssoondiscoveredthattheycouldusecertainfeaturesof
the host computer’s operating system to give them unauthorized access to
computer resources. Others took advantage of users who had relatively little
computerknowledge tosubstitutetheirownprograms—writtenfortheir own
purposes—in place of common or innocuous utilities. These unsophisticated
users wouldrun what theythoughtwas theirusual software only tofind their
files erased, to have their account passwords stolen, or to suffer other
unpleasant consequences. Such “Trojan horse” programs or “Trojans,” so
dubbedfortheir metaphoricalresemblanceto theancient Greekgift tothecity
of Troy, remain a significant, and growing, threat to computer users today.
Viruses and the PC revolution
What we now think of as true computer viruses first appeared, according to
RobertSlade,soon afterthefirst personal computers reached the mass market
intheearly1980s.Otherresearchersdatetheadventofvirusprogramsto1986,
with the appearance of the “Brain” virus.Whichever date hasthe better claim,
the link between the virus threat and the personal computer is not
coincidental.
The new mass distribution of computers meant that viruses could spread to
many more hosts than before, when a comparatively few, closely guarded
mainframe systems dominated the computing world from their bastions in
large corporations and universities.Nor did the individual userswho bought
PCs have much use for the sophisticated securitymeasures needed to protect
sensitive data in those environments. As further catalyst, virus writers found
it relatively easy to exploit some PC technologies to serve their own ends.
Boot-sector viruses
Early PCs, for example, “booted” or loaded their operating systems from
floppy disks. The authors of the Brain virus discovered that they could
substitute their own program for the executable code present on the boot
sector of every floppy disk formatted with Microsoft’s MS-DOS, whether or
notitincluded systemfiles. Userstherebyloadedthe virusintomemoryevery
time they started their computers with any formatted disk in their floppy
drives. Once in memory,a virus cancopy itself toboot sectors on otherfloppy
or hard disks. Those who unintentionally loaded Brain from an infected
floppy found themselves reading an ersatz “advertisement” for a computer
consulting company in Pakistan.
With that advertisement, Brain pioneered another characteristic feature of
modern viruses: the payload. The payload is the prank or malicious behavior
that,iftriggered,causeseffectsthatrangefromannoyingmessagestodata
destruction. It’s the virus characteristic that draws the most attention—many
virus authors now write their viruses specifically to deliver their payloads to
as many computers as possible.