— Trojan horse—A Trojan horse is a program that usually claims to perform
one function (such as a game) but also does something completely different
(such as corrupting data on your hard disk). Many different types of Trojan
horses get attached to systems. The effects of these programs range from
minor user irritation to total destruction of the computer’s file system.
Trojan horses are sometimes used to exploit systems by creating user
accounts on systems so that an unauthorized user can gain access or
upgrade her privilege level. Trojans are also commonly used to enlist
computers for a distributed denial of service (DDoS) attack without the
knowledge of the system owner.