According to an update by Guy Rosen, VP of Product Management at Facebook regarding the security issue discovered and fixed two weeks ago, fewer people were impacted than originally thought. Of the 50 million people whose access tokens Facebook believed were affected, about 30 million actually had their tokens stolen. Here’s their explanation of how it happened:

First, the attackers already controlled a set of accounts, which were connected to Facebook friends. They used an automated technique to move from account to account so they could steal the access tokens of those friends, and for friends of those friends, and so on, totaling about 400,000 people. In the process, however, this technique automatically loaded those accounts’ Facebook profiles, mirroring what these 400,000 people would have seen when looking at their own profiles. That includes posts on their timelines, their lists of friends, Groups they are members of, and the names of recent Messenger conversations. Message content was not available to the attackers, with one exception. If a person in this group was a Page admin whose Page had received a message from someone on Facebook, the content of that message was available to the attackers.

The attackers used a portion of these 400,000 people’s lists of friends to steal access tokens for about 30 million people. For 15 million people, attackers accessed two sets of information – name and contact details (phone number, email, or both, depending on what people had on their profiles). For 14 million people, the attackers accessed the same two sets of information, as well as other details people had on their profiles. This included username, gender, locale/language, relationship status, religion, hometown, self-reported current city, birthdate, device types used to access Facebook, education, work, the last 10 places they checked into or were tagged in, website, people or Pages they follow, and the 15 most recent searches. For 1 million people, the attackers did not access any information.

People can check whether they were affected by visiting Facebook’s Help Center. In the coming days, Facebook claims they will send customized messages to the 30 million people affected to explain what information the attackers might have accessed, as well as steps they can take to help protect themselves, including from suspicious emails, text messages, or calls.

Customized messages that people will see depending on how they were impacted.

This attack did not include Messenger, Messenger Kids, Instagram, WhatsApp, Oculus, Workplace, Pages, payments, third-party apps, or advertising or developer accounts.

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