Meeting Melissa - and other such pests
If you have surfed the Internet in the last week or received email then you will have come across all the shock and panic of the "Melissa" virus. Company IT departments have "been at work all weekend making their systems safe" from this latest scourge. This week's article is about viruses and how to spot them and, most importantly, how to avoid them!
Virus Types
There are different forms of computer virus ranging from those that display silly messages to those that cause major damage to the data stored on your computer.
Thousands of viruses and their numerous strains exist. They started life as "practical jokes" to play on those people who are/were not computer literate. With the advent of virus detection programs, the race was on between the virus programmer and those that write the detection programs. As soon as one virus is detected, another strain is created in the hope that a particular virus will escape detection.
Some viruses actually involve some clever programming and you have to wonder why talented programmers spent time and effort in this direction rather than writing something useful. It IS the preying on the innocent that seems to appeal and with the Internet, whole new races of "newbies" are waiting to be caught.
Of the types, there are (in very simple terms), two main types. Trojan horses, which are programs "hidden" within other programs and viruses that attach themselves to legitimate programs and force execution of the virus code.
Most viruses allow "breeding". This means that as part of their programming they will copy themselves to other programs on the host machine. Thus the spread of infection become ever greater.
How do viruses work?
Virus programs that latch onto other programs often employ the technique of adding a line of instructions at the beginning of program which then "jumps" to the end of the infected program so that the virus code itself is run. Once run, execution then passes back to the main program. In English, this could look something like this....
Jump to virus start Start of main program
...
...
...
End of main program
Start of virus code
Carry out destruction sequence
Display silly message
Trash hard disk
Copy code to another program
End of virus code
Return to start of main program
So if the main program were a graphic file viewer, the program would start by jumping to the start of the virus code, do the destructive code then return to the main program.
Often the person running the program is unaware that anything is amiss until long after the program has been run.
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