Intrusions are unauthorized attempts to enter a network or computer. These attempts may take several different forms, but the goal is the same: to gain entry so the attacker can either destroy or steal data, gain control of equipment or gather information about the network for later use.
While the old-fashioned methods are still used, attackers now will frequently use trojans attached to innocuous looking files to introduce tools for later exploitation. Trojans can be introduced in a number of ways. They can be posted to newsgroups or IRC channels where users will download them, sent through email as attachments or simply placed on a website for the unwary to acquire. They can even be installed on the victim's machine without their knowledge when they visit a web site. Using weaknesses in the security of the victim's web browser, the attacker can bypass the existing security controls and drop a trojan into a directory on the victim's hard drive.