Russian hackers were inside a Ukrainian telecoms giant’s system for months before launching a massive cyberattack in December, according to a report. Illia Vitiuk, head of the Security Service of Ukraine’s (SBU) cybersecurity department, told Reuters that hackers had breached Kyivstar—Ukraine’s largest telecommunications provider—since at least May last year. He said the hack led to “disastrous” damage and was aimed at gathering intelligence as well as delivering a psychological blow. “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,” Vitiuk said, noting that Kyivstar is a wealthy private company which invested heavily in cybersecurity. Hacking group Solntsepek claimed responsibility for the attack, but Vitiuk said he’s “pretty sure” it was conducted by a unit of Russia’s military intelligence known as Sandworm.