Computer viruses and worms are malicious software programs that spread from computer to computer and interfere with operations. Viruses spread through email attachments and downloads, while worms replicate by exploiting security vulnerabilities to copy themselves to other systems on a network. Common types are executable viruses, boot sector viruses, and macro viruses like the damaging Melissa virus of 1999. Prevention methods include installing antivirus software, keeping systems updated, and using firewalls. Trojan viruses differ in that they do not self-replicate but perform unauthorized actions like deleting or blocking access to data when activated. The first major computer worm was the Morris worm of 1988, which disrupted many systems and led to the conviction of its creator.

1. COMPUTER VIRUSES AND WORMS
#1 VIRUS
Computer viruses are small software programs that are designed to spread from one computer to
another and to interfere with computer operation.
 A computer virus might corrupt or delete data on your computer, use your email program to
spread itself to other computers, or even erase everything on your hard disk.
 Computer viruses are often spread by attachments in email messages or instant messaging
messages. That is why it is essential that you never open email attachments unless you know
who it's from and you are expecting it.
 Viruses can be disguised as attachments of funny images, greeting cards, or audio and video
files.
 Computer viruses also spread through downloads on the Internet. They can be hidden in
illicit software or other files or programs you might download.
Typical things that some current Personal Computer (PC) viruses do:
 Display a message
 Erase files
 Scramble data on a hard disk
 Cause erratic screen behavior
 Halt the PC
 Many viruses do nothing obvious at all except
spread!
Types:
1. Executable Viruses
 Traditional viruses
 Pieces of code attached to a legitimate program
 Run when the legitimate program gets executed
 Loads itself into memory and looks around to see if it can find any other programs
on the disk
2. Boot Sector Viruses
 Traditional Virus
 Infect the boot sector on floppy disks and hard disks
 By putting its code in the boot sector, a virus can guarantee it gets executed
 Load itself into memory immediately, and it is able to run whenever the computer is
on

2. MELISSA VIRUS:
 March 1999
 The Melissa virus was the fastest-spreading virus ever seen
 Someone created the virus as a Word document uploaded to an Internet news group
 People who downloaded the document and opened it would trigger the virus
 The virus would then send the document in an e-mail message to the first 50 people in
the person's address book
 Took advantage of the programming language built into Microsoft Word called VBA
(Visual Basic for Applications)
 The Melissa virus is a macro virus that was spread through email attachments in 1999.
 The Melissa virus may also be known as Melissa, Simpsons, Kwyjibo or Kwejeebo.
PREVENTION:
 Updates
o Windows has a feature called Critical Updates
o An entire branch of Microsoft dedicated to staying one step ahead of the
hackers
o When a possible security loophole is fixed, you need to download the patch right
away
 Anti-Viruses
 More secure operating systems
o e.g. UNIX
 Here are some actions you can take:
o Install an antivirus program.
o Don't open email attachments unless you're expecting them.
o Keep your computer updated.
o Use a firewall. Windows Firewall (or any other firewall) can help alert you to
suspicious activity if a virus or worm attempts to connect to your computer
TROJAN VIRUS
Trojans are malicious programs that perform actions that have not been authorized by the
user. These actions can include:
 Deleting data
 Blocking data
 Modifying data
 Copying data
 Disrupting the performance of computers
or computer networks
 Unlike computer viruses and worms, Trojans are not able to self-replicate
 How Trojans can impact you:
BACKDOOR: A backdoor Trojan gives malicious users remote control over the infected
computer. They enable the author to do anything they wish on the infected computer –
including sending, receiving, launching and deleting files, displaying data and rebooting

3. the computer. Backdoor Trojans are often used to unite a group of victim computers to
form a botnet or zombie network that can be used for criminal purposes.
EXPLOIT: Exploits are programs that contain data or code that takes advantage of a
vulnerability within application software that’s running on your computer
 A very important distinction from true viruses is that they do not replicate
themselves, as viruses do. Trojans contain malicious code, that, when triggered,
cause loss, or even theft, of data. In order for a Trojan horse to spread, you must, in
effect, invite these programs onto your computers--for example, by opening an email
attachment.
#2 WORMS
Worm - is a self-replicating program, similar to a computer virus. A virus attaches itself to, and
becomes part of, another executable program; however, a worm is self-contained and does not
need to be part of another program to propagate itself.
WORMS – is a small piece of software that uses computer networks and security holes to replicate
itself. A copy of the worm scans the network for another machine that has a specific security hole. It
copies itself to the new machine using the security hole, and then starts replicating from there, as
well.
They are often designed to exploit the file transmission capabilities found on many computers.
MORRIS WORM: The first worm to attract wide attention, the Morris worm, was written by Robert
Tappan Morris, who at the time was a graduate student at Cornell University.
It was released on November 2, 1988. The level of replication created system loads that not only
brought it to the attention of system administrators, but also disrupted the target computers. It was
guessed that the cost in "potential loss in productivity" caused by the worm and efforts to remove it
ranged at each system from $200 to more than $53,000
Morris himself was convicted under the US Computer Crime and Abuse Act and received three years’
probation, community service and a fine in excess of $10,000.
ROBERT TAPPAN MORRIS IS NOW A PROFESSOR AT MIT