Ask HN: How does FB know my phone number?

5 points·dandare··8 comments

I have never shared my phone number with FB, not even for the 2FA purpose. I use a FB app on my Android phone, but it also never requested a permission to use my phone number. When I checked on FB before the leak there was no phone number in my account information. Yet my phone number, along with all my other credentials, is in the recent leak. What am I missing?

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The weakest link is some of your contacts that opted (because that is the default) to upload their phone contacts to a Facebook service. Through those phone contacts, Facebook correlates the contact name with your Facebook account.

Ah fuck. I was wondering about this because I was in the same boat as OP. Even weirder - I checked my profile settings and my phone number was listed even though I'm certain never added it and stopped using mobile years ago.

Someone you know signed into WhatsApp, which uploads all your contacts to Facebook without their permission.

From their privacy policy ("Services" includes FB):

"We receive information about you from other users. For example, when other users you know use our Services, they may provide your phone number, name, and other information (like information from their mobile address book) just as you may provide theirs."

How can this be legal? GDPR compliant?

I may be wrong. But, based on this solution https://stackoverflow.com/questions/58128546/how-to-get-the-... permission is not required to get phone number--if we use play services API

Maybe I'm wrong, but I imagine if you use the Facebook app on your phone, then Facebook can grab your number and attach it as metadata in your account.

Did you not read the small print when you agreed to become a human centipede?

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