Several users’ computers are no longer responding normally and sending out spam email to the users’
entire contact list. This is an example of which of the following?

A.
Trojan virus

B.
Botnet

C.
Worm outbreak

D.
Logic bomb

Explanation:
A worm is similar to a virus but is typically less malicious. A virus will usually cause damage to the system
or files whereas a worm will usually just spread itself either using the network or by sending emails.
A computer worm is a standalone malware computer program that replicates itself in order to spread to
other computers. Often, it uses a computer network to spread itself, relying on security failures on the
target computer to access it. Unlike a computer virus, it does not need to attach itself to an existing
program. Worms almost always cause at least some harm to the network, even if only by consuming
bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.
Incorrect Answers:
A: In computers, a Trojan horse is a program in which malicious or harmful code is contained inside
apparently harmless programming or data in such a way that it can get control and do its chosen form of
damage, such as ruining the file allocation table on your hard disk. In one celebrated case, a Trojan horse
was a program that was supposed to find and destroy computer viruses. A Trojan horse may be widely
redistributed as part of a computer virus. In this question, no actual damage was done to the computers.
Emails were sent to spread the worm.
B: A botnet is a collection of Internet-connected programs communicating with other similar programs in
order to perform tasks. This can be as mundane as keeping control of an Internet Relay Chat (IRC)
channel, or it could be used to send spam email or participate in distributed denial-of-service attacks. The
word botnet is a combination of the words robot and network. The term is usually used with a negative or
malicious connotation.
Computers can be co-opted into a botnet when they execute malicious software. This can be
accomplished by luring users into making a drive-by download, exploiting web browser vulnerabilities, or
by tricking the user into running a Trojan horse program, which may come from an email attachment.
This malware will typically install modules that allow the computer to be commanded and controlled by
the botnet’s operator. Many computer users are unaware that their computer is infected with bots.Depending on how it is written, a Trojan may then delete itself, or may remain present to update and
maintain the modules. Botnets can be used to send spam emails but this would be done by someone
controlling the computers to target an individual or entity with the spam. A worm would use its own
programming to send emails to everyone in a contact list with the aim of spreading itself.
D: A logic bomb is a piece of code intentionally inserted into a software system that will set off a malicious
function when specified conditions are met. For example, a programmer may hide a piece of code that
starts deleting files should they ever be terminated from the company.
Software that is inherently malicious, such as viruses and worms, often contain logic bombs that execute
a certain payload at a pre-defined time or when some other condition is met. This technique can be used
by a virus or worm to gain momentum and spread before being noticed. Some viruses attack their host
systems on specific dates, such as Friday the 13th or April Fool’s Day. Trojans that activate on certain
dates are often called “time bombs”.
To be considered a logic bomb, the payload should be unwanted and unknown to the user of the
software. As an example, trial programs with code that disables certain functionality after a set time are
not normally regarded as logic bombs. A logic bomb may contain a worm but it is the worm that is
sending the emails.

http://en.wikipedia.org/wiki/Computer_worm
http://searchsecurity.techtarget.com/definition/Trojan-horse
http://en.wikipedia.org/wiki/Botnet
http://en.wikipedia.org/wiki/Logic_bomb