You Gave Facebook Your Number For Security. They Used It For Ads

Add “a phone number I never gave Facebook for targeted advertising” to the list of deceptive and invasive ways Facebook makes money off your personal information. Contrary to user expectations and Facebook representatives’ own previous statements, the company has been using contact information that users explicitly provided for security purposes—or that users never provided at all—for targeted advertising.

A group of academic researchers from Northeastern University and Princeton University, along with Gizmodo reporters, have used real-world tests to demonstrate how Facebook’s latest deceptive practice works. They found that Facebook harvests user phone numbers for targeted advertising in two disturbing ways: two-factor authentication (2FA) phone numbers, and “shadow” contact information.

Two-Factor Authentication Is Not The Problem

First, when a user gives Facebook their number for security purposes—to set up 2FA, or to receive alerts about new logins to their account—that phone number can become fair game for advertisers within weeks. (This is not the first time Facebook has misused 2FA phone numbers.)

But the important message for users is: this is not a reason to turn off or avoid 2FA. The problem is not with two-factor authentication. It’s not even a problem with the inherent weaknesses of SMS-based 2FA in particular. Instead, this is a problem with how Facebook has handled users’ information and violated their reasonable security and privacy expectations.

There are many types of 2FA. SMS-based 2FA requires a phone number, so you can receive a text with a “second factor” code when you log in. Other types of 2FA—like authenticator apps and hardware tokens—do not require a phone number to work. However, until just four months ago, Facebook required users to enter a phone number to turn on any type of 2FA, even though it offers its authenticator as a more secure alternative. Other companies—Google notable among them—also still follow that outdated practice.

Even with the welcome move to no longer require phone numbers for 2FA, Facebook still has work to do here. This finding has not only validated users who are suspicious of Facebook's repeated claims that we have “complete control” over our own information, but has also seriously damaged users’ trust in a foundational security practice.

Until Facebook and other companies do better, users who need privacy and security most—especially those for whom using an authenticator app or hardware key is not feasible—will be forced into a corner.

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