Hackers claim to have compromised and published private messages from at least 81,000 Facebook users’ accounts.

But Facebook claims that no hack has taken place, and instead blames browser companies, such as Google and Mozilla, for the data leak saying that users have installed malicious software that is leaking their details to the ‘hackers’.

Facebook executive Guy Rosen said: “We have contacted browser-makers to ensure that known malicious extensions are no longer available to download in their stores.

“We have also contacted law enforcement and have worked with local authorities to remove the website that displayed information from Facebook accounts.”

Browser developers, such as Google and Mozilla, which control 78% of desktop browsing, allow third-party extensions. As yet, it remains unknown which extensions may be the culprits guilty of leaking the data.

Browser extensions are widely used and add common features to browsers like Chrome and Firefox, such as spell checking.

By allowing an extension to see everything they type, users are allowing a plugin to see their passwords, account details and information. It is easy to see how such an extension could be comprised and the data used maliciously by hackers to misuse someone’s online banking.

Users’ private Facebook accounts being sold for pennies

The hackers have told the BBC Russian Service that they took the details, which they are trying to sell, from a total of 120 million accounts.

The BBC’s Russian Service investigated the claim and said it found account details were indeed for sale for as little as eight pence each. However, the advert hawking the accounts has since been taken offline.

It appears the majority of accounts affected are from Ukraine and Russia but, according to the BBC’s analysis, some British users could be affected too.

Cyber security company Digital Shadows examined the claim on behalf of the BBC and confirmed that more than 81,000 of the profiles posted online as a sample contained private messages.

The BBC Russian Service made contact with five Russian Facebook users whose private messages had been exposed and had them confirm it was their account. One account included holiday snaps, another intimate messages between lovers and one containing complaints about a son-in-law.

According to the BBC, cyber experts said that if these extensions are to blame then the browsers’ developers may share some of the responsibility for failing to test the programs for safety.