Attributing computer network intrusions has grown in importance as cyber penetrations across sovereign borders have become commonplace. Although advances in technology and forensics have made machine attribution easier in recent years, identifying states or others responsible for cyber intrusions remains challenging. This essay provides an overview of the attribution problem and its international legal dimensions and argues that states must develop accountable attribution mechanisms for international law to have practical value in this sphere.
Scope and Nature of the Attribution Problem
When it comes to cyber activity, we live in what Lucas Kello calls a state of “unpeace”—“mid-spectrum rivalry lying below the physically destructive threshold of interstate violence, but whose harmful effects far surpass the tolerable level of peacetime competition.”Footnote 1 More than thirty nations are now capable of effectively using cyber tools as weapons. Moreover, as the devices and techniques for deploying them become more advanced, identifying the boundary between cyber effects that do not rise to the level of armed force and more damaging cyber weapons that do remains contentious and difficult.
Some states and nonstate actors use cyber tools to strike with impunity, knowing (or at least strongly suspecting) that their digital attacks will not prompt a response, certainly not a kinetic response. The exfiltration of data and intellectual property has been going on for some time, at great cost to governments and private industry. More recent and increasingly sophisticated forms of offensive hacking are capable of causing more significant harm, even catastrophic damage, such as shutting down financial systems, sabotaging critical infrastructure, and scrambling communications. These activities all place a premium on knowing the source of the cyber intrusion, so that states can respond accordingly.
The inability to identify the source potentially increases the risks of confusion and escalation. For example, when the United States released an unclassified summary of its Department of Defense Cyber Strategy in September 2018, attention focused on its commitment to “defend forward to disrupt or halt malicious cyber activity at its source, including activity that falls below the level of armed conflict.”Footnote 2 In other words, U.S. Cyber Command will interdict cyber threats outside combat zones. Yet some see the “defending forward” idea as putting the U.S. military on an offensive, rather than defensive, footing. The recent shift in U.S. cyber policy deepens a cyber variant on a classic security dilemma between the United States and other states: As one state takes steps to defend itself in cyberspace, it inadvertently threatens other states with what appears to be offensive action. In practice, “defending forward” can look like attacking forward to those experiencing an intrusion. One implication is an increased tendency to escalate conflicts.Footnote 3 In an environment where escalation fears are on the rise, the possibility that cyber intrusions could spike destructive and even destabilizing conflicts between states places a premium on confident attribution of cyber intrusions.
Technical and Practical Challenges
Attribution is defined as “identifying the agent responsible for the action.”Footnote 4 Attribution of cyber intrusions can be challenging. States often use technical tools to hide their responsibility and/or rely on nonstate proxies to carry out cyber activities for them.Footnote 5 However, significant technological strides in attributing cyber events in the last decade have made the task “more nuanced, more common, and more political than has typically been acknowledged.”Footnote 6 The nuance involves combining experienced and disciplined technical operators with the intuition and judgment of intelligence professionals. The political aspect includes assessing what is at stake in making the attribution judgment, starting with the damage incurred, whether physical, financial, or reputational.Footnote 7 As such, attribution is often expressed in degrees of certainty, and it requires input from a range of actors and sources, including technical forensics, human intelligence, signals intelligence, history, and diplomatic relations.Footnote 8
Understanding the components of attribution is essential for shaping a legal and policy strategy to deter harmful cyber intrusions in the future.Footnote 9 As cyber intrusions have proliferated in recent years, states have invested in doing attribution well and, as a result, deterring or at least discouraging states and other cyber intruders.Footnote 10 When attribution is done badly or not at all, states lose credibility and likely effectiveness in dealing with those who would harm the state and its citizens. These risks hold for state-on-state interactions across the spectrum of cyber operations—from espionage to destructive attacks on infrastructure.
Although identifying the machines responsible for a cyber intrusion is no longer a difficult task for the most advanced states,Footnote 11 identifying the persons, organizations, or states that are legally responsible for the cyberattack remains challenging.Footnote 12 The problems derive from technical means of deception and anonymity, but they are also due to the vagaries of the process of fixing responsibility for cyberattacks within the international community and the malleability and open-endedness of the few attribution rules that currently exist in international law.Footnote 13
Legal Challenges in Attributing an Unlawful Cyber Intrusion
The law of attribution for cyber intrusions aims to identify and place responsibility for internationally wrongful acts. The customary law of state responsibility for violations of international law, and with it the law of attribution, is drawn largely from the long-term work of the International Law Commission (ILC) and its Draft Articles on Responsibility of States for Internationally Wrongful Acts.Footnote 14 The ILC rules were commended to the member states by the UN General Assembly in 2001 and have become the authoritative guidepost for public international cyber law. Drawing from the ILC rules, the starting point in the cyber context is that “a State bears international responsibility for a cyber-related act that is attributable to the State and that constitutes a breach of an international legal obligation.”Footnote 15 In essence, states are responsible for the wrongful cyber-related acts of their own officials, agents, contractors, nonstate actors, and other states, to the extent they actually control the operations.Footnote 16 States do not escape legal responsibility for internationally wrongful acts by perpetrating them through proxies.Footnote 17
Outside an armed conflict, international law forbids cyber intrusions that violate the prohibition on intervention.Footnote 18 Based on the international law principle of sovereignty, the principle forbids coercive intervention by cyber means.Footnote 19 The consensus among experts is that state-on-state cyber intrusions that are not coercive but are “detrimental, objectionable, or otherwise unfriendly”Footnote 20 are not international legal violations. As confirmed by the ICJ in the Nicaragua judgment, “the element of coercion … forms the very essence of … prohibited intervention.”Footnote 21
Yet international law has never had a precise definition of “coercion.” According to a consensus among the cyber experts who contributed to Tallinn 2.0, “coercion is not limited to physical force, but rather refers to an affirmative act designed to deprive another State of its freedom of choice … to force that State to act in an involuntary manner or involuntarily refrain from acting in a particular way.”Footnote 22 A state compels another state by, for example, providing cyber training or supplying malware to a terrorist group operating in the compelled state.Footnote 23 Yet defining the full range of cyber conduct that qualifies as “coercion” has been more difficult. The International Group of Experts (IGE) that provided the analysis in Tallinn 2.0 could only agree on the anodyne statement “that as a general matter, States must act as reasonable States would in the same or similar circumstances when considering responses to them.”Footnote 24 The IGE elaborated:
Reasonableness is always context dependent. It depends on such factors as, inter alia, the reliability, quantum, directness, nature (e.g., technical data, human intelligence), and specificity of the relevant available information when considered in light of the attendant circumstances and the importance of the right involved. These factors must be considered together. Importantly in the cyber context, deficiencies in technical intelligence may be compensated by, for example, the existence of highly reliable human intelligence.Footnote 25
Recognizing that customary international law has not developed a set of understandings or recognized state practice on what level of attribution is acceptable or necessary for establishing state responsibility for cyber actions, the IGE concluded that “States may agree between themselves to a rule of responsibility specific to a cyber act or practice.”Footnote 26 The result would be lex specialis to the extent the rule conflicts directly with general principles of state responsibility.Footnote 27 Similar uncertainty surrounds the degree of certainty on attribution a state should achieve before asserting the legal right to take self-help measures in response that would otherwise violate international law.
Deciding How and When a “Victim” State May Respond
If a state is victimized by an internationally wrongful act below the use of force threshold, the “State may be entitled to take countermeasures, whether cyber in nature or not.”Footnote 28 Countermeasures are responses that otherwise would violate international law, designed to prevent a responsible state from continuing its unlawful cyber intervention.Footnote 29 Countermeasures require prior notice to the offending state, and they must have as their purpose inducing compliance with international law.Footnote 30 Punitive countermeasures are forbidden.Footnote 31 Short of countermeasures, states may respond to cyber intrusions through retorsions, acts that are “unfriendly” but lawful.Footnote 32 Examples include protests, denying access to state resources, and economic sanctions.Footnote 33
The Tallinn 2.0 experts agreed that
in the context of unilateral self-help measures, the reality is that States must make ex ante determinations with respect to attribution of a cyber operation to another State before responding. … [T]he State may be faced with a situation to which it may have to respond in an extremely short time frame, without recourse to the full range of information that might be available in the non-cyber context.Footnote 34
The IGE opined that “as a general matter the graver the underlying breach … , the greater the confidence ought to be in the evidence relied upon by a State considering a responseFootnote 35 … because the robustness of permissible self-help responses … grows commensurately with the seriousness of the breach.”Footnote 36 However, the severity of the cyber intrusion directed at an injured state is also relevant, so that a state confronted with “low-level cyber operations that are merely disruptive” may be expected to amass more evidence for attribution than a state victimized by “devastating cyber operations and needing to respond immediately to terminate them.”Footnote 37
In a similar vein, the time it takes to produce a high confidence attribution judgment can limit the lawful responses to cyber operations. Mistaken attribution can lead to an unlawful response even if the state made a reasonable attribution judgment and implemented countermeasures.Footnote 38 If a state victimized by an internationally wrongful cyber intrusion engages in countermeasures and ends up being wrong about state attribution, the victimized state has committed an internationally wrongful act.Footnote 39 On the other hand, if the victim state waits until it has high confidence in its attribution of a state's responsibility for the intrusion, any countermeasures may be construed as punishment, a form of reprisal forbidden under international law.Footnote 40 As a result, cyber deterrence may be undermined because the legally less risky but weak self-help retorsion responses to an intrusion are unlikely to deter similar cyber intrusions in the future.
Prospects for International Legal Regulation
The short-term prospects for international legal regulation of cyber attribution are not encouraging. The failure of a state to provide persuasive proof of attribution is not itself an internationally wrongful act. Nor are there burdens of proof or additional legal criteria for establishing attribution. The 2015 United Nations Group of Governmental Experts (GGE) report noted that accusations of wrongful acts by states “should be substantiated,”Footnote 41 but the GGE gave no indication of which or how much evidence would suffice or even count. The U.S. view, articulated by State Department Legal Adviser Brian Egan in November 2016, is that “a State acts as its own judge of the facts and may make a unilateral determination with respect to attribution of a cyber operation to another State. … [T]here is no international legal obligation to reveal evidence on which attribution is based prior to taking appropriate action.”Footnote 42
States are likewise not obligated to provide evidence of attribution when responding to another state's cyber intrusions.Footnote 43 While the IGE acknowledged the value in such a disclosure requirement, it found insufficient state practice and opinio juris to recognize “an established basis under international law for such an obligation.”Footnote 44 The IGE noted that the highly classified nature of such attribution assessments is the primary reason for the absence of customary international law on this important point.Footnote 45
Although attribution is necessarily probabilistic, the process serves its purpose if it convinces the responsible state (and victim state's citizens) that a response to the cyber intrusion is called for.Footnote 46 The fact that attribution judgments draw on many different sources of information has one major temporal implication—early judgments made with less information are generally less believable than later judgments made with more information. Continuing investigation may reveal additional useful information, which may (or may not) reinforce attribution judgments made earlier. Over time, an international consensus may develop on the minimum level of involvement needed to declare that a state is legally responsible for a cyber operation.
States are better able to attribute cyber intrusions than they were a decade ago. Yet the technical environment is so dynamic that new tools constantly both improve and occlude attribution capabilities. Spoofing and other challenges can greatly complicate attribution and a response when an immediate response is required, particularly when a state is the suspected perpetrator. As cyber international relations now stand, a few states benefit from the absence of express cyber norms on what suffices to attribute state responsibility for cyber exploitation because they have the most offensive cyber capabilities. However, in general, those states are also the most vulnerable to cyber intrusions. Meanwhile, the disparity between states that are strong and weak at attribution results in the equivalent of an arms race between advances in detection versus detection evasion. Evasion is getting easier faster, so states that do not have advanced attribution capabilities can reliably invest in hiding themselves.Footnote 47
As the most advanced cyber states recognize the risks of cyber escalation, those states have good reason to become more transparent about attribution in service of the mutual restraint that could be gained by sharing attribution information. But to date, state concerns about revealing intelligence sources and methods counsel against transparency.Footnote 48
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