Facebook: User shadow data, including phone numbers may be used by advertisers

The worst suspect is a disconcerting reality, Facebook admitted that advertisers were able to access phone numbers of its users for enhanced security.

Researchers from two American universities discovered that that phone numbers given to Facebook for two-factor authentication were also used for advertising purposes.

“These findings hold despite all the relevant privacy controls on our test accounts being set to their most private settings,” reads the study published by the researchers. 

“Most worrisome, we found that phone numbers uploaded as part of syncing contacts — that were never owned by a user and never listed on their account – were in fact used to enable PII-based advertising,”

The study investigates the channels used by advertisers can gather personally identifying information (PII) from Facebook, WhatsApp and Messenger services.

The contact lists uploaded to the Facebook platforms could be used by advertisers that once extracted the personal information can leverage it to target people in their networks.

The experts speculate Facebook is using a hidden layer of details it has about its users, like phone numbers used for 2FA authentication, that they called “shadow contact information.”

The study supported concerns that Facebook uses “shadow” sources of data not given to the social network for the purpose of sharing to make money on advertising.

“We use the information people provide to offer a better, more personalized experience on Facebook, including showing more relevant ads.” a spokeswoman told Gizmodo that first reported the news.

Facebook continues to face a severe crisis due to the way it manages data of its users, the Cambridge Analytica case has shocked the world about the way the social network giant has shared the information of its unaware users with third party companies.

At the time of writing, Facebook’s Guy Rosen, VP of Product Management announced that attackers exploited a vulnerability in the “View As” feature to steal Facebook access tokens of 50 Million Users.

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Pierluigi Paganini

(Security Affairs – Facebook, privacy)

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