A tool or set of tools used by an attacker in order to compromise a system, gain the highest level of privilege, and then hide their activity.

A hacker security tool that captures passwords and message traffic to and from a computer. A collection of tools that allows a hacker to provide a backdoor into a system, collect information on other systems on the network, mask the fact that the system is compromised, and much more. Rootkit is a classic example of Trojan horse software.