Russian hackers were inside Ukrainian telecoms giant Kyivstar’s system from at least May last year in a cyberattack which crippled its services in December, Ukraine’s top cyber spy said.

In an interview with Reuters published Thursday, Illia Vitiuk, head of the Security Service of Ukraine’s cybersecurity department, said: “This attack is a big message, a big warning, not only to Ukraine, but for the whole Western world to understand that no one is actually untouchable,” adding it wiped “almost everything,” including thousands of virtual servers and PCs.

The attack caused more than 24.3 million Kyivstar customers to lose phone reception, with banks reporting disruptions to their services and Ukrainians in the country’s eastern war zone being left without a connection. Vitiuk has attributed the attack to Sandworm, a Russian military intelligence cyberwarfare unit which has been linked to cyberattacks in Ukraine and elsewhere.

“For now, we can say securely, that they were in the system at least since May 2023,” Vitiuk said, adding, “I cannot say right now, since what time they had … full access: probably at least since November.”

In a video statement in December, Kyivstar CEO Oleksandr Komarov said: “Unfortunately, the war with Russia has several dimensions. One of them is in cyberspace.”