Facebook Breach Leaves Half a Billion Users Hanging on the Line

It seems that there is simply no end to a long series of Facebook’s privacy blunders. This time, a security researcher has stumbled upon an unprotected server hosting several huge databases containing phone numbers of 419 million Facebook users from different countries. Judging by the screenshot included in an article by Techcrunch, this looks like another case of a misconfigured MongoDB server exposed to the Internet without any access controls. Each record in those databases contains a Facebook user’s unique ID that can be easily linked to an existing profile along with that user’s phone number. Some also contained additional data like name, gender or location.

Facebook has denied that it has anything to do with those databases, and there is no reason to doubt that; the sheer negligence of the case rather points to a third party lacking even basic security competence, perhaps a former Facebook marketing partner. This is by far not the first case of user data being harvested off Facebook by unscrupulous third parties, perhaps the biggest one being the notorious Cambridge Analytica scandal of early 2018. After that, Facebook has disabled access to users’ phone numbers to all its partners, so the data leaked this time is perhaps not the most current.

Still, the huge number of affected users and the company’s apparent inability to find any traces of the perpetrators clearly indicate that Facebook hasn’t done nearly enough to protect their users’ privacy in recent times. Until any further details emerge, we can only speculate about the leak itself. What we could do today, however, is to try and figure out what users can possibly do to protect them from this leak and to minimize the impact of future similar data breaches.

First of all, the most common advice “don’t give your phone number to Facebook and the likes” is obviously not particularly helpful. Many online messaging services (like WhatsApp or Telegram) use phone numbers as the primary user identities and simply won’t work without them. Others (like Google, Twitter or even your own bank) rely on phone numbers to perform two-factor authentication. Second, for hundreds of millions of people around the world, this advice comes too late – their numbers are already at the disposal of spammers, hackers, and other malicious actors. And those guys have a few lucrative opportunities to exploit them…

Besides the obvious use of these phone numbers for unsolicited advertising, they can be used to expose people who use pseudonyms on social media and link those accounts to real people – for suppressing political dissent or simply to further improve online user tracking. Alas, the only sensible method of preventing this breach of privacy is to use a separate dedicated phone number for your online services, which can be cumbersome and expensive (not to mention that it had to be done before the leaks!)

Unfortunately, in some countries (including the USA), leaked phone numbers can also be used for SIM swap attacks, where a fraudster tricks a mobile operator to issue them a new SIM card with the same number, effectively taking full control over your “mobile identity”. With that card, they can pose as you in a phone call, intercept text messages with one-time passwords and thus easily take over any online service that relies on your mobile number as the means of authentication.

Can users do anything to prevent SIM swap attacks? Apparently not, at least, until mobile operators are forced by governments to collaborate with police or banks on fighting this type of fraud. Again, the only sensible way to minimize its impact is to move away from phone-based (not-so-)strong authentication methods and adopt a more modern MFA solution: for example, invest in a FIDO2-based hardware key like Yubikey or at least switch to an authenticator app like Authy. And if your bank still offers no alternative to SMS OTP, maybe today is the right time to switch to another bank.

Remember, in the modern digital world, your phone number is the key to your online identity. Keep it secret, keep it safe!


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