A computer hacker in Russia has advertised for sale what appear to be passwords and login details from the social networking site Facebook.
Cybersecurity experts say any concerned users should change their passwords immediately.
According to the US-based cybersecurity company iDefense, the passwords first appeared for sale on a Russian internet site in February.
The BBC reports the advertisement was posted by a user named Kirllos, who offered to sell Facebook logins in batches of 1000 for between $US25 and $US45 per batch.
Kirllos claims to have the account details of 1.5 million Facebook users.
Cyber experts say such information can be sold for a variety of criminal uses.