Volt Typhoon had access to the US electrical grid for 300 days.

Researchers found that the Volt Typhoon APT was able to directly access the US electrical grid for 300 days. The reseraches found that the group had access to the grid betwee Feburary and November of 2023. Volt Typhon has been responsible for a number of high profile attacks on US infrastructure.

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https://www.securityweek.com/chinas-volt-typhoon-hackers-dwelled-in-us-electric-grid-for-300-days/