Facebook data leak: How to check if hackers accessed your account, stole information

Could hackers have been able to see the last person you cyberstalked, or that party photo you were tagged in? According to Facebook, the unfortunate answer is "yes."

On Friday, the social network said fewer users were affected in a security breach it disclosed two weeks ago than originally estimated — nearly 30 million, down from 50 million. In additional good news, the company said hackers weren't able to access more sensitive information like your password or financial information. And third-party apps weren't affected.

Still, for users already uneasy about the privacy and security of their Facebook accounts after a year of tumult , the details that hackers did gain access to — gender, relationship status, hometown and other info — might be even more unsettling.

Facebook has been quick to let users check exactly what was accessed. But beyond learning what information the attackers accessed, there's relatively little that users can do — beyond, that is, watching out for suspicious emails or texts. Facebook says the problem has been fixed.

The company set up a website that its 2 billion global users can use to check if their accounts have been accessed, and if so, exactly what information was stolen. It will also provide guidance on how to spot and deal with suspicious emails or texts. Facebook will also send messages directly to those people affected by the hack.

On that page, following some preliminary information about the investigation, the question "Is my Facebook account impacted by this security issue?" appears midway down. It will also provide information specific to your account if you're logged into Facebook.

Facebook said the hackers accessed names, email addresses or phone numbers from these accounts. For 14 million of them, hackers got even more data — basically anything viewable on your account that any of your friends could see, and more. It's a pretty extensive list: user name, gender, locale or language, relationship status, religion, hometown, self-reported current city, birthdate, device types used to access Facebook, education, work, the last 10 places you checked into or were tagged in, your website, people or Pages you follow and your 15 most recent searches.

An additional 1 million accounts were affected, but hackers didn't get any information from them.

The company isn't giving a breakdown of where these users are, but says the breach was "fairly broad." It plans to send messages to people whose accounts were hacked.

Facebook said the FBI is investigating, but asked the company not to discuss who may be behind the attack. The company said it hasn't ruled out the possibility of smaller-scale attacks that used the same vulnerability.

The company said it has fixed the bugs and logged out affected users to reset those digital keys.

Facebook vice president Guy Rosen said in a Friday call with reporters that the company hasn't ruled out the possibility that other parties might have launched other, smaller scale efforts to exploit the same vulnerability before it was disabled.

Patrick Moorhead, founder of Moor Insights & Strategy, said the breach appeared similar to identity theft breaches that have occurred at companies including Yahoo and Target in 2013.

"Those personal details could be very easily be used for identity theft to sign up for credit cards, get a loan, get your banking password, etc.," he said. "Facebook should provide all those customers free credit monitoring to make sure the damage is minimized."

Thomas Rid, a professor at the Johns Hopkins University, also said the evidence, particularly the size of the breach, seems to point to a criminal motive rather than a sophisticated state operation, which usually targets fewer people.

"This doesn't sound very targeted at all," he said. "Usually when you're looking at a sophisticated government operation, then a couple of thousand people hacked is a lot, but they usually know who they're going after."