A leaker says they are offering information on more than 500 million Facebook Inc users - including phone numbers and other data - virtually for free.
Alon Gal, co-founder of Israeli cybercrime intelligence firm Hudson Rock, said on Saturday the database appears to be the same set of Facebook-linked telephone numbers that has been circulating in hacker circles since January and whose existence was first reported by tech publication Motherboard.
Reuters was not immediately able to vet the information, which is being offered for a few euros' worth of digital credit on a well-known site for low-level hackers, but Gal said he had verified the authenticity of at least some of the data by comparing it against phone numbers of people he knew.
Other journalists say they have also been able to match known phone numbers to the details in the data dump.
An attempt by Reuters to reach the leaker over the messaging service Telegram was not immediately successful. Facebook did not return messages seeking comment.
Motherboard's story earlier this year cited Facebook as saying the leaked data appeared to be the fruit of a bug that the company fixed in August 2019.
Gal told Reuters that Facebook users should be alert to "social engineering attacks" by people who may have obtained their phone numbers or other private data in the coming months.
News of the latest leak was first reported by Business Insider.