=== Possible obligation not to conduct cyber operations against other states’ critical infrastructure ===
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> ''[https://www.icj-cij.org/files/case-related/70/070-19860627-JUD-01-00-EN.pdf Military and Paramilitary Activities in and against Nicaragua] (Nicaragua v US)'' (Merits)  ICJ Rep 14, para 207, citing ICJ, ''[https://www.icj-cij.org/files/case-related/51/051-19690220-JUD-01-00-EN.pdf 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 [http://www.un.org/en/ga/search/view_doc.asp?symbol=A/RES/68/243 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.
Finally, even if the GGE did have the mandate and ability to identify relevant rules of custom, it most certainly did not do so with regard to operations against critical national infrastructure. It is manifest from the formulation cited above that the governmental experts did not consider any intentional cyber operation against critical national infrastructure to be internationally unlawful. Rather, they included the phrase “contrary to its obligations under international law”, confirming that on their view, such an operation would only violate international law if there is an ''additional'' obligation that would be breached by the operation in question.<ref>Cf. 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>
In sum, the preferred view is that a standalone rule prohibiting cyber operations against critical national infrastructure has not emerged in international law thus far. As such, the incident in the scenario cannot be described as infringing this supposed obligation.