Explanation/Reference:
Explanation:
A trojan horse is any code that appears to have some useful purpose but contains code that has a malicious or harmful purpose imbedded in it. It is non-self-replicating malware that often includes a trapdoor as a means to gain access to a computer system bypassing security controls.
Incorrect Answers:
A: A Virus is a malicious program that can replicate itself and spread from one system to another. It does not appear to be harmless; its sole purpose is malicious intent often doing damage to a system.
B: A Worm is similar to a Virus but does not require user intervention to execute. Rather than doing damage to the system, worms tend to self-propagate and devour the resources of a system.
D A trapdoor is a means to bypass security by hiding an entry point into a system. Trojan Horses often have a trapdoor imbedded in them.
References:
Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, New York, 2013, pp. 1213, 1214
http://en.wikipedia.org/wiki/Trojan_horse_(computing)
http://en.wikipedia.org/wiki/Computer_virus
http://en.wikipedia.org/wiki/Computer_worm
http://en.wikipedia.org/wiki/Backdoor_(computing)