More than 6,000 Coinbase users had funds stolen from their accounts after hackers used a vulnerability in Coinbase’s SMS-based two-factor authentication system to breach accounts.

The intrusions took place earlier this year, between March and May, the exchange said in a data breach notification letter it has filed with US state attorney general offices.

“The third party took advantage of a flaw in Coinbase’s SMS Account Recovery process in order to receive an SMS two-factor authentication token and gain access to your account,” Coinbase said.