Methods and apparatus providing computer and network security utilizing probabilistic policy reposturing

A system defines at least one key event to be monitored by at least one agent, and creates a graphical model for the at least one key event. The system observes the at least one key event. The system infers a degree of attack on the computer system based on an observation of the at least one key event in conjunction with a result of an effect the at least one key event has on the graphical model. The system then adjusts a security policy based on an output of the graphical model.