| |
-
A computer virus is a piece
of program designed and written to make additional copies of itself and
spread from location to location, typically without user knowledge or
permission.
-
Viruses are written by
programmers with malicious intent to annoy computer users. There are
different categories of viruses - some of these may corrupt or destroy
data files stored in the hard disk. These include:
- Boot-sector viruses -
The boot sector is that part of the system software containing most of the
instructions for starting up the computer system. The boot-sector virus
replaces these instructions with some of its own. Once the system is turned on,
the virus is loaded into main memory before the operating system. From there
it is in a position to infect all files. Any diskette that is used in the drive
of the computer then becomes infected. When that diskette is moved to another
computer, the virus continues spreading.
- File viruses - attach
to executable program files (filenames with extensions .com, .exe) - those that
actually begin a program. When the program is run, the virus starts working,
trying to get into main memory and infecting other files.
- Macro viruses - Many
application programs such as Word and Excel embed miniature programs, known as
macros, inside files and templates. Macros are executed each time the file or
template is opened. Macro viruses attach their macros to files and templates.
Therefore when an application loads the file and executes the instructions in it,
the first instructions to execute are those of the virus.
- Trojan horses - covertly
place illegal, destructive instructions in the middle of a computer program. Once
the program is run, the Trojan horse becomes active. Trojans do not replicate
themselves like other viruses.
-
Worms are similar to viruses in
that they make copies of themselves, but differ in that they need not attach to
particular files or sectors at all. Once a worm is executed, it seeks other systems
- rather than parts of systems - to infect, then copy its code to them. Typically
worms slow down computer systems.
-
Typically, viruses spread by:
- Introducing infected disks into
the computer.
- Opening infected files attached to
e-mail messages.
- Sharing infected files over the
intranet i.e. the internal campus computer network.
- Downloading infected files from
ftp (file transfer protocol) sites on the Internet.
|
