Adversaries continue to penetrate U.S. information technology networks, and in many cases, they have infiltrated the online environment, jeopardizing the confidentiality, integrity, and availability of enterprise information and systems. A multitude of network-related incidents have shown that the fortress model of securing information systems—hard on the outside, soft on the inside—falsely assumes that the boundary between hard and soft can prevent all types of penetration. Given this vulnerability of boundaries, network attacks are pervasive, and nefarious code is present even in the face of system sweeps to discover and clean readily apparent malware.