“Article 2(4) of the Charter of the United Nations provides that \u201cAll Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.\u201d At the same time, international law recognizes that there are exceptions to this rule. For example, in the exercise of its inherent right of self-defense a State may use force that is necessary and proportionate to respond to an actual or imminent armed attack. This is true in the cyber context just as in any other context.\n <\/p>\n

“[…] the international law prohibition on coercively intervening in the core functions of another State (such as the choice of political, economic, or cultural system) applies to State conduct in cyberspace. For example, \u201ca cyber operation by a State that interferes with another country\u2019s ability to hold an election\u201d or that tampers with \u201canother country\u2019s election results would be a clear violation of the rule of non-intervention.\u201d Other States have indicated that they would view operations that disrupt the fundamental operation of a legislative body or that would destabilize their financial system as prohibited interventions.\n <\/p>\n

There is no international consensus among States on the precise scope or reach of the non-intervention principle, even outside the context of cyber operations. Because States take different views on this question, DoD lawyers examining any proposed cyber operations must tread carefully, even if only a few States have taken the position publicly that the proposed activities would amount to a prohibited intervention.\n <\/p>\n

“For cyber operations that would not constitute a prohibited intervention or use-of-force, the Department believes there is not sufficiently widespread and consistent State practice resulting from a sense of legal obligation to conclude that customary international law generally prohibits such non-consensual cyber operations in another State\u2019s territory. This proposition is recognized in the Department\u2019s adoption of the \u201cdefend forward\u201d strategy: \u201cWe will defend forward to disrupt or halt malicious cyber activity at its source, including activity that falls below the level of armed conflict.\u201d The Department\u2019s commitment to defend forward including to counter foreign cyber activity targeting the United States\u2014comports with our obligations under international law and our commitment to the rules-based international order.\n <\/p>\n

The DoD OGC view, which we have applied in legal reviews of military cyber operations to date, shares similarities with the view expressed by the U.K. Government in 2018. We recognize that there are differences of opinion among States, which suggests that State practice and opinio juris are presently not settled on this issue. Indeed, many States\u2019 public silence in the face of countless publicly known cyber intrusions into foreign networks precludes a conclusion that States have coalesced around a common view that there is an international prohibition against all such operations (regardless of whatever penalties may be imposed under domestic law).\n <\/p>\n

Traditional espionage may also be a useful analogue to consider. Many of the techniques and even the objectives of intelligence and counterintelligence operations are similar to those used in cyber operations. Of course, most countries, including the United States, have domestic laws against espionage, but international law, in our view, does not prohibit espionage per se even when it involves some degree of physical or virtual intrusion into foreign territory. There is no anti-espionage treaty, and there are many concrete examples of States practicing it, indicating the absence of a customary international law norm against it. In examining a proposed military cyber operation, we may therefore consider the extent to which the operation resembles or amounts to the type of intelligence or counterintelligence activity for which there is no per se international legal prohibition.\n <\/p>\n