1. Computer Systems Viruses

2. Virus A virus is a program which can destroy or cause damage to data stored on a computer. It’s a program that must be run in order to infect a computer. Viruses attach themselves to other programs to ensure this happens

3. Virus Symptoms ► Displaying unwanted messages ► Unusual visual or sound effects ► Loss of data from backing storage ► Unexpected rebooting ► Unwanted generation of emails

4. Virus Actions ► Corrupt or delete data ► Disable computer by changing O/S ► Silly messages or sounds displayed ► Generate enough emails to crash servers ► Record key strokes & send data back to virus writer ► Use your computer to attack other computers.

5. File Virus ► This attaches itself to an executable file such as an application program or a game. ► They either replace or insert malicious code into the files. ► When you run the program, the virus instructions are also carried out. ► Usually infect files with.com or.exe extentions.

6. Boot Sector Virus ► Every hard & floppy disk has an area called the ‘boot sector’. ► This contains boot code. These are files your computer uses every time you power up. ► By infecting this code, a boot sector virus guarantees it gets run.

7. Macro Virus ► Application programs (Word, Excel etc) let the user create and embed ‘Macros’. ► A macro simply ‘records’ a users actions so that repetitive tasks can be automated. ► A macro virus causes a malicious sequence of actions to be performed when the document is opened.

8. Virus Techniques Replication ► A virus inserts copies of itself into other program files. ► Each time the infected program is run, it reproduces itself, copying itself into another program.

9. Virus Techniques Camouflage ► A virus can disguise itself to avoid detection by adding fake instructions to its code. ► Anti-virus software is unable to spot the unique pattern of code (signature) which identifies the virus. ► Each time the virus runs, it changes the false code.

10. Virus Techniques Watching ► A virus can lie and wait or watch for a particular action or date before it is activated. ► Meantime, it replicates!

11. Virus Techniques Delivery ► The method used by the virus to enter the computer system. ► Infected disks used to be a common method, now viruses spread in seconds through file downloads from the Internet or via email.

12. Worm ► A worm can transfer themselves to other computers without needing to be transferred as part of a host program. They are usually transferred via email. ► Once in a computer, they simply replicate themselves filling up memory and using processor resources. This prevents programs being loaded and slows down a computer drastically.

13. Trojan ► A program that appears to be safe but hidden inside can be a worm or a virus. ► You may download a game or picture but once you run the file, the virus or worm gets to work.

14. Anti-virus (Checksum) ► All files are binary data. A checksum does a calculation on this binary data. ► Each time the program is run, the calculation is re-performed. ► If the calculation produces a different checksum, the code has been altered. Possibly by a virus.

15. Anti-virus (Signature) ► A virus is a program like any other. ► It contains sections of unique code, its signature. ► Anti-virus software uses a table, which has to be regularly updated, containing virus signatures.

16. Anti-virus (MRM) ► Memory Resident Monitoring. Memory Resident software loads into the computers memory when it’s started up and stays there until it’s shuts down again. ► Anti-virus software does this and constantly monitors the computer for viruses.

17. Anti-virus (Heuristics) ► This looks for code that is triggered by time or date events, for code that searches for.com or.exe files or files that try to directly write to disk without going through the normal O/S procedure. ► If it looks like a virus and behaves like a virus, then it probably is a virus.