Microsoft has confirmed that the passwords of some of its Hotmail users were posted to a website, but had since been taken down.
According to Reuters, the company did not specify how many of its email users were affected, but reports suggest that over 10,000 passwords were exposed.
"We are aware that some Windows Live Hotmail customers' credentials were acquired illegally by a phishing scheme and exposed on a website," a Microsoft spokesman said.
All the account details have reportedly been removed from the unnamed website. Microsoft says it has blocked the affected Hotmail addresses and is helping users regain access their accounts.
BBC News has reportedly seen an additional list of over 20,000 email login detaills from providers including Yahoo, AOL and Gmail, as well as Hotmail.
The details were published on the same website as the original list of around 10,000 Hotmail passwords.
According to the BBC, some of the accounts seem to be fake, old or unused, however, many are said to be genuine.