The drawbacks and dangers of active defense

The growing prevalence of cyber-attacks on states, businesses, and individuals has raised new and urgent questions about the legal framework that governs states´ capacity to respond such attacks. An issue that has proven particularly vexing is what actions a state may take in response to attacks that fall into the gap between the actions that constitute a prohibited “use of force” under Article 2(4) of the UN Charter and the “armed attacks” to which a state has a right to respond with force in self defense under Article 51. Intrusions that constitution an illegal “use of force” but do not meet the “armed attack” threshold for triggering a legal forceful response-sometimes known as “below the threshold” cyber-operations-are extraordinarily common. Indeed, nearly all cyber-attacks by one state on another fall below the “armed attack” threshold. If states cannot legally use their right to self-defense to respond to such unlawful attacks, what can they do? There is a growing consensus that the answer can be found in countermeasures doctrine. Yet countermeasures doctrine was never intended to be applied to actions that constitute uses of force. There is good reason for this: if forceful countermeasures were allowed, there would be a serious danger that the system restricting illegal use of force would spin out of control. Improper countermeasures are inevitable, and escalation of confl ict only a matter of time. This paper outlines the legal principles governing the use of force in international affairs, describes the exceptions to the broad prohibition on the use of military force, outlines the doctrine of countermeasures, and-in its key contribution to the debate-outlines reasons for concern about aggressive countermeasures. The paper concludes by briefl y considering non-forceful responses that states may take in response to cyber-attacks.