Security patterns are now starting to be accepted by industry. Security patterns are useful to guide the security design of systems by providing generic solutions that can stop a variety of attacks but it is not clear to an inexperienced designer what pattern should be applied to stop a specific attack. They are not useful either for forensics because they do not emphasize the modus operandi of the attack. To complement security patterns, we have proposed a new type of pattern, the misuse pattern. This pattern describes, from the point of view of the attacker, how a type of attack is performed (what units it uses and how), defines precisely the context of the attack, analyzes the ways of stopping the attack by enumerating possible security patterns that can be applied for this purpose, and describes how to trace the attack once it has happened by appropriate collection and observation of forensics data. We present here a model that characterizes the precise structure of this type of pattern.