Russian hackers have been inside the system of Ukrainian telecom giant Kyivstar since at least May last year, in a cyber attack that should serve as a “major warning” to the West, the head of Ukraine’s cyber espionage service told Reuters.

The hack, one of the most dramatic since the Russian invasion began nearly two years ago, has shut down services provided by Ukraine’s largest telecommunications company since Dec. 12. to about 24 million users.

In an interview, Ilya Vityukhead of the Cybersecurity Department of the Security Service of Ukraine (SBU), revealed exclusive details of the hack, which he said had caused “disaster” and was aimed at inflicting a psychological blowas well as gather information.

“This attack is a big message, a big warning, not only for Ukraine, but for the entire Western world, to understand that no one is really immune,” he said. He pointed out that Kyivstar is a wealthy private airline that has invested large sums in cyber security.

The attack swept “almost everything,” including thousands of servers and PCshe said, describing it as possibly the first example of a catastrophic cyberattack that “completely destroyed the core of the telecommunications company.”

During the investigation, the SBU found that hackers had probably tried to infiltrate Kyivstar in March or even earlier, he said in a Zoom interview on December 27.

“For now, we can safely say that they have been in the system since at least May 2023,” he said. “I can’t say right now, since when they had … full access: probably at least since November.”

The SBU estimates that the hackers, with the level of access they gained, may have been able to steal personal information, learn phone locations, intercept SMS messages and perhaps hijack accounts on the Telegram platform, it said.

A Kyivstar spokesperson said the company is working closely with the SBU to investigate the attack and will take all necessary measures to eliminate potential future risks.

Vityuk stated that the SBU helped Kyivstar restore its systems within days and fend off new cyberattacks.

“After the big breach, there were a number of new attempts aimed at doing more damage to the phone company,” he said.

Kyivstar is the largest of Ukraine’s three main telecommunications companies, and about 1.1 million Ukrainians live in small towns and villages where no other telecommunications companies operate, Vityuk said.