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Facebook's Download Your Information (DYI) tool has occasionally been criticized for not offering enough transparency into user-account data, but it recently revealed more than it intended when a bug led it to leak the contact information of some six million users.

In an advisory posted on Friday, Facebook's security team explained that the code the social network uses to make friend recommendations inadvertently caused the email addresses and phone numbers of potential contacts to be associated with other users' account data.

If those users then used the DYI tool, the wrongly added contact information would be included in the download, whether or not the users were actually friends with the owners of the addresses or numbers in question.

"After review and confirmation of the bug by our security team, we immediately disabled the DYI tool to fix the problem and were able to turn the tool back on the next day once we were satisfied that the problem had been fixed," Facebook's White Hat staff wrote.

In all, the security team has concluded that the addresses or phone numbers of around six million Facebook users were leaked in this way – a figure equivalent to about 0.54 per cent of the social network's global user base.

Zuck's security bods have determined that each individual email address or phone number was typically only included in a download once or twice, meaning it was only leaked to one person. In addition, Facebook assures us that only other people could have had access to the data – as opposed to developers or advertisers – and no other financial or personal information was disclosed.

The bug was first brought to Facebook's attention by an independent security researcher, whom the social site's security staff say has already been paid a bug bounty for his efforts. In addition, Facebook has notified its regulators in the US, Canada, and Europe of the incident and is in the process of notifying affected users via email.

Equally important, although the social network has downplayed the severity of the leak and it doubts that the bug was ever exploited for malicious purposes, it's still really, really, really sorry about the whole thing.

"It's still something we're upset and embarrassed by, and we'll work doubly hard to make sure nothing like this happens again," the White Hat team wrote. "Your trust is the most important asset we have, and we are committed to improving our safety procedures and keeping your information safe and secure." ®

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