On 23 December 2015, cyberattackers disrupted electricity supply to approximately 230,000 customers in Ukraine's Ivano-Frankivsk Oblast for one to six hours — the first publicly confirmed successful cyberattack on civilian power grid infrastructure anywhere in the world. US-CERT (IR-ALERT-H-16-056-01), the Ukrainian SBU, and security researchers attributed the attack to Sandworm (GRU Unit 74455), who used spear-phishing to compromise three Ukrainian regional electricity distribution companies, deployed the BlackEnergy malware and KillDisk wiper, and launched a telephony denial-of-service to hamper restoration. A follow-on attack on 17 December 2016 (Industroyer/Crash Override) struck the Kiev transmission substation.