The U.S. government has concluded for the first time that a December power outage in Ukraine affecting 225,000 customers was the result of a cyberattack.
Security experts had already widely concluded that the downing of utilities in western Ukraine on December 23 was due to an attack, which is believed to be the first-known successful cyber-intrusion to knock out a power grid.
The U.S. Department of Homeland Security acknowledged that for the first time on February 25 in a published alert from its Industrial Control Systems Cyber Emergency Response Team.
The department said its assessment was based on interviews with six Ukrainian organizations affected by the blackout.
During the attack, hackers remotely switched breakers in a way that cut power after installing malware, it said.
The attackers are also believed to have spammed the Ukraine utility's customer-service center with phone calls in order to prevent real customers from communicating about their downed power.