The threats that adversaries pose can be characterized along two dimensions—the sophistication of the intrusion and the damage it causes. Though these two are often related, they are not the same. Sophistication is needed to penetrate good cyberdefenses, and the damage an intrusion can cause depends on what the adversary does after it has penetrated those defenses. As a general rule, a greater availability of resources to the adversary (e.g., more money, time, talent) will tend to increase the sophistication of the intrusion that can be launched against any given target and thus the likelihood that the adversary will be able to penetrate the target’s defenses.

Two important consequences follow from this discussion. First, because nation-state adversaries can bring to bear enormous resources to conduct an intrusion, the nation-state threat (perhaps conducted through intermediaries) is the most difficult to defend against. Second, stronger defenses reduce the likelihood but cannot eliminate the possibility that even less sophisticated adversaries can cause significant damage.

A Range of Possibilities

The discussion below focuses primarily on cyberattacks as the primary policy concern of the United States, and addresses cyber exploitation as necessary.


The central policy question is how to achieve a reduction in the frequency, intensity, and severity of cyberattacks on U.S. computer systems and networks currently being experienced and how to prevent the far more serious attacks that are in principle possible. To promote and enhance the cybersecurity of important U.S. computer systems and networks (and the information contained in or passing through these systems and networks), much attention has been devoted to passive defense—measures taken unilaterally to increase the resistance of an information technology system or network to attack. These measures include hardening systems against attack, facilitating recovery in the event of a successful attack, making security more usable and ubiquitous, and educating users to behave properly in a threat environment.6

Passive defenses for cybersecurity are deployed to increase the difficulty of conducting the attack and reduce the likelihood that a successful attack will have significant negative consequences. But experience and recent history have shown that they do not by themselves provide an adequate degree of cybersecurity for important information systems and networks.

A number of factors explain the limitations of passive defense. As noted in previous NRC reports,7 today’s decision-making calculus regarding cybersecurity


As an example, see NRC, Toward a Safer and More Secure Cyberspace, 2007.


National Research Council, Cybersecurity Today and Tomorrow: Pay Now or Pay Later, The National Academies Press, Washington, D.C., 2002; NRC, Toward a Safer and More Secure Cyberspace, 2007.

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