DNA testing site MyHeritage has said the company has been hacked.
The company said the breach affected all accounts up to and including to October 26, 2017.
"There has been no evidence that the data in the file was ever used by the perpetrators," said Omer Deutsch, the company's chief information security officer in a blog post, adding that the company has not seen any indication that the accounts had been compromised.
The company didn't name the security researcher.
MyHeritage allows users to search a vast trove of genealogy records to build up family trees and discover the histories of their ancestors.
The company said it was informing the relevant authorities, including European regulators, which recently lauded the introduction of the the new EU's General Data Protection Regulation. The law, known as GDPR, came into effect on May 25 and allows regulators to fine companies that violate the new law up to four percent of the firm's global revenue for the previous year.