Cyber attack on U.S. Power grid networks

A March 2019 NERC 'Lessons Learned' bulletin disclosed that a low-impact denial-of-service event affected an unnamed western US utility's perimeter Cisco firewalls when an attacker exploited a known vulnerability that had not been patched. The incident caused intermittent loss of communications between control-room operators and field SCADA equipment for several hours.

No loss of generation or load; operators retained visibility through redundant communication paths. The incident prompted updated guidance on firewall-firmware patch hygiene at regulated entities.