Viruses And The Pc Revolution - McAfee VIRUSSCAN 5.1 User Manual

Preface
Some of these students soon discovered that they could use certain features of
the host computer's operating system to give them unauthorized access to
computer resources. Others took advantage of users who had relatively little
computer knowledge to substitute their own programs—written for their own
purposes—in place of common or innocuous utilities. These unsophisticated
users would run what they thought was their usual software only to find their
files erased, to have their account passwords stolen, or to suffer other
unpleasant consequences. Such "Trojan horse" programs or "Trojans," so
dubbed for their metaphorical resemblance to the ancient Greek gift to the city
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
Robert Slade, soon after the first personal computers reached the mass market
in the early 1980s. Other researchers date the advent of virus programs to 1986,
with the appearance of the "Brain" virus. Whichever date has the 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 users who bought
PCs have much use for the sophisticated security measures 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
not it included system files. Users thereby loaded the virus into memory every
time they started their computers with any formatted disk in their floppy
drives. Once in memory, a virus can copy itself to boot sectors on other floppy
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, if triggered, causes effects that range from annoying messages to data
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.
User's Guide ix