Risk and financial advisory solutions provider Kroll on Friday disclosed that one of its employees fell victim to a “Highly sophisticated” SIM swapping attack.

“Specifically, T-Mobile, without any authority from or contact with Kroll or its employee, transferred that employee’s phone number to the threat actor’s phone at their request,” it said in an advisory.

SIM swapping, while generally a benign process, could be exploited by threat actors to fraudulently activate a SIM card under their control with a victim’s phone number.

The disclosure arrives days after Bart Stephens, the co-founder of Blockchain Capital, filed a lawsuit against an anonymous hacker who stole $6.3 million worth of crypto in an alleged SIM swap attack.

Earlier this month, the U.S. Department of Homeland Security’s Cyber Safety Review Board urged telecommunications providers to employ stronger security protocols to prevent SIM swapping, including by providing options for customers to lock their accounts and enforce stringent identity verification checks.

The frequency of SIM swapping attacks is a reminder for users to move away from SMS-based two-factor authentication and switch to phishing-resistant methods to secure online accounts.