The scenario notes that the cyber operation against company X had caused significant inconvenience to many households in State A. The blackout must also have resulted in economic damage to company X and other actors on State A’s territory, likely including the State itself. However, there is no indication of actual physical damage having occurred or of any injury to individuals as a result of the operation. Therefore, the principal legal question is whether such forms of interference may be categorized as a use of force inconsistent with Article 2(4) of the UN Charter. As noted, the law is unsettled in this regard and a clear conclusion cannot be made at present.
In any event, the characterization of an incident of this nature as amounting to a use of force would be of limited consequence in the present scenario. This is because even if a particular act by a State qualifies as prohibited force, the victim State and its allies may only respond in self-defence if the said act is additionally of sufficient gravity to amount to an “armed attack”,<ref> Charter of the United Nations (adopted 26 June 1945, entered into force 24 October 1945) 1 UNTS 16 (UN Charter) Art 51.</ref> and even then, the permitted response is further limited by the conditions of necessity and proportionality.<ref> See, eg, ''Military and Paramilitary Activities in and against Nicaragua (Nicaragua v US)'' (Merits)  ICJ Rep 14, para 194; ''Legality of the Threat or Use of Nuclear Weapons Case'' (Advisory Opinion)  ICJ Rep 226, para 41; ''Oil Platforms (Iran v US)''  ICJ Rep 161, para 43.</ref> However, the lack of destructive effects in State A strongly militates against the qualification of the cyber operation by State B as an “armed attack” under international law.<ref> ''Military and Paramilitary Activities in and against Nicaragua (Nicaragua v US)'' (Merits)  ICJ Rep 14, para 195 (holding that an operation must be characterized by sufficient “scale and effects” in order to qualify as an “armed attack”); but see Tallinn Manual, commentary to rule 71, para 12 (noting that some experts held “the view that a cyber operation directed against a State’s critical infrastructure that causes severe, albeit not destructive, effects would qualify as an armed attack“).</ref>
Moreover, the fact that the source of the disruption was only identified ''after'' the disruptive effects had been addressed means that at that point, it could no longer be said that a use of force in self-defence by State A or by alliance O was necessary to repel an ongoing attack by State B.<ref> Cf G Nolte and A Randelzhofer, ‘Article 51’ in B Simma et al (eds), ''The Charter of the United Nations: A Commentary'' (3rd edn, OUP 2012) vol II, 1426–27, para 60 (noting that the use of force in self-defence is limited to ending the attack so that the specific impulse from which the attack emerged is no longer present).</ref> Of course, State A would still be entitled to call upon the UN Security Council to qualify the cyber operation as having amounted to a “breach of the peace” and to decide on measures under Chapter VII of the UN Charter.<ref>See Charter of the United Nations (adopted 26 June 1945, entered into force 24 October 1945) 1 UNTS 16 (UN Charter) Art 39.</ref>
However, the spectrum of unilateral responses available to State A under international law is otherwise identical to those available in response to violations of international law other than the prohibition of force. It is thus arguably unnecessary to conclusively determine if the cyber operation against State A did in fact cross the threshold of Article 2(4) of the UN Charter, as long as the relevant conduct breached other applicable international legal rules. This is what the remainder of the analysis turns to.
In its 2015 report, the UN group of governmental experts agreed on a formulation that “[a] State should not conduct or knowingly support ICT activity contrary to its obligations under international law that intentionally damages critical infrastructure or otherwise impairs the use and operation of critical infrastructure to provide services to the public“.<ref> UN GGE 2015 'Report of the Group of Governmental Experts on Developments in the Field of Information and Telecommunications in the Context of International Security report' (22 July 2015) UN Doc A/70/174, para. 13(f).</ref> This raises the question whether a cyber operation such as the one described in this scenario infringes an obligation not to attack critical national infrastructure of other States.
However, it is uncertain whether such an obligation can be said to exist in the present state of international law. Three points should be made in this regard. First of all, cyber operations against critical infrastructure have recently become a very frequent occurrence in the international practice.<ref> See, eg, US, Director of National Intelligence James Clapper, [https://www.dni.gov/files/documents/Unclassified_2015_ATA_SFR_-_SASC_FINAL.pdf Worldwide Threat Assessment of the US Intelligence Community] (26 February 2015) (“foreign actors are reconnoitering and developing access to U.S. critical infrastructure systems, which might be quickly exploited for disruption if an adversary’s intent became hostile.”); FO Hampson and M Sulmeyer (eds), ''Getting Beyond Norms'' (CIGI 2017) 6 (“Disrupting or damaging critical infrastructures that provide services to the public has become customary practice — the new normal. In the past two years and since the GGE agreement, there have been an alarming number of harmful incidents targeting critical infrastructures around the world, ranging from power systems to telecommunications systems to transportation systems to financial systems.”).</ref> Of course, the frequency of particular type of conduct, even if it “amount[s] to a settled practice”, does not by itself suffice to establish a new permissive rule of customary law.<ref> ''Military and Paramilitary Activities in and against Nicaragua (Nicaragua v US)'' (Merits)  ICJ Rep 14, para 207, citing ICJ, ''North Sea Continental Shelf Cases (Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands)''  ICJ Rep 3, para 77.</ref> However, the fact that most of these instances have not been accompanied by specific condemnations by those States in a position to react to them, suggests that equally, no corresponding prohibitive rule has emerged thus far.<ref> Cf. also ECCC, Case No 002/19-09-2007-EEEC/ OICJ (PTC38), Decision on the Appeals Against the Co-Investigative Judges Order on Joint Criminal Enterprise (JCE) (20 May 2010), para 53 (“A wealth of State practice does not usually carry with it a presumption that ''opinio juris'' exists”).</ref>
Secondly, although UN GGE reports are based on the consensus of governmental experts selected on the basis of equitable geographical representation,<ref> UNGA Res 68/243 (27 December 2013) UN Doc A/RES/68/243, para 4.</ref> their views cannot be said to automatically reflect customary international law. The mandate of the UN GGE is decidedly not focussed on the identification of customary legal rules; rather, it includes the proposing and promoting of “[v]oluntary, ''non-binding norms'' of responsible State behaviour”.<ref> UN GGE 2015 'Report of the Group of Governmental Experts on Developments in the Field of Information and Telecommunications in the Context of International Security report' (22 July 2015) UN Doc A/70/174, para 10. On the distinction between cyber rules and cyber norms, see K Mačák, ‘From Cyber Norms to Cyber Rules: Re-engaging States as Law-makers’ (2017) 30 LJIL 877, 877–99.</ref> As such, the legal valence of any normative statements found in one of the reports must be understood as minimal to none.