Malicious browser extensions could be behind a compromise of at least 81,000 Facebook accounts which were put up for sale on the dark web, according to reports.

Those behind the attack told the BBC Russian Service that they had access to 120 million accounts, although this has been branded “unlikely” by Digital Shadows, whose researchers were called in to investigate.

In fact, the seller, “FBSaler,” provided a total dataset to reporters of around 257,000 profiles. Just 81,000 are certain to have been compromised, as private messages were included. The remaining 176,000 may have simply had profile information like names, addresses, contact numbers, and interests taken because accounts were left wide open by users.

The accounts are not thought to be linked to the Cambridge Analytica scandal, or the more recent breach of 30 million accounts which occurred after attackers obtained access tokens.

“The method used to obtain the accounts remains unconfirmed, though Facebook believe malicious browser extensions could have been used. Facebook have still not been definitive about this, though it said it had contacted browser makers to ensure that known malicious extensions are no longer available to download in their stores,” said Digital Shadows.

“A rogue survey application as used by Kogan is known to have worked in the past; however, account takeovers achieved through credential harvesters, for example, are also a possibility. While a variety of separate breaches may have been used to compile the dataset, it is more likely a single approach was used given the consistency of the data in the dump.”

The largest number of profiles (30%) are Ukrainian, followed by Russia (9%), although users from the US, UK and Brazil are also said to be represented.

“Regardless of attribution, motives and the method of collection, the exposure of private messages where people share information they would not usually post publicly on their Facebook feeds is a potentially worrying development,” the firm warned. “Sensitive information may be used for extortion of identity fraud, while it’s not unheard of for individuals to share financial information such as banking details over private messages.”

The accounts were originally for sale for around $0.10 each on the BlackHat SEO forum, although the report claimed the advert has since been taken down, according to the BBC.