Facebook users may have shared their personal information – including phone numbers – on an online hacking forum.
More than 500 million users’ data was stolen due to a security issue in 2018, according to the company.
But it was shared on a hacking forum on Saturday.
Facebook said it has since fixed the problem, but experts have warned that even old data is still a risk of fraud.
The huge data leak affects data from 533 million accounts in 106 countries.
Around 11 million people are affected in the UK.
The details that could be shared include birthdays, names, locations, and in some cases emails.
Alon Gal, chief technology officer for internet security company Hudson Rock, said insider : “A database of this size, containing private information such as the phone numbers of many Facebook users, would certainly lead bad actors to use the data to conduct social engineering attacks (or hacking attempts).
“People who register with a reputable company like Facebook trust them with their data, and Facebook should treat the data with the greatest possible respect.
“Users whose personal information has been leaked are a major breach of trust and should be treated accordingly.”