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Your Facebook friends could be leaving you vulnerable to major privacy invasions

A computer scientist explains how you have way less control of your data than Mark Zuckerberg says.

Mark Zuckerberg, of Facebook.
Moving fast — and vacuuming up your data.
Justin Sullivan/Getty Images

Facebook finally released a tool last week allowing users to check if their personal data was compromised by Cambridge Analytica, the shady British firm that used such data to “psychologically profile” potential voters. I took a deep breath, clicked, and was greeted with good news: “Based on our available records, neither you nor your friends logged in to ‘This Is Your Digital Life’” — the app that the company used to lure Facebook users into revealing personal data. So my data was not compromised.

There’s an aside in that message that ought to alarm every Facebook user: the phrase “neither you nor your friends.” And indeed, many Facebook users who checked their status were greeted by a different message: “You don’t appear to have logged into ‘This Is Your Digital Life’ … [h]owever, a friend of yours did.” That means their data was collected, although they (wisely) had not personally enabled the app.

That’s because even a user with stringent privacy control practices, but who has friends who were less careful, could have ended up with Cambridge Analytica collecting private information like their phone number, who their family members are, all the places they’ve “checked in,” and which groups they’ve joined — including those whose very names and subjects might reveal private information, such as support groups for health conditions.

And really, who can trust that none of their hundreds of friends is careless? All of us have that one uncle who responds to “Nigerian prince” emails, or the high school friend who has never met a personality quiz they didn’t take. Unfortunately, the way Facebook’s privacy controls were set up prior to 2014 made it effectively impossible for even the most conscientious Facebook user to protect themselves against the data leakage one friend could cause.

And despite many rounds of revisions and overhauls over the years, Facebook privacy controls still leave many things outside users’ control. Too many users still don’t realize just how vulnerable their data is to their friends’ third-party apps.

Mark Zuckerberg was scheduled to meet this week with the European Commission vice president in charge of digital issues, Andrus Ansip. Here’s some of the problematic history — and continuing problematic behavior — that Ansip, not to mention US regulators, might want to zero in on:

Facebook opened the privacy floodgates with apps

Remember how annoying the days of unfettered apps on Facebook were? Farmville and other games sent a deluge of notifications to friends of players (“Sally Smith sent you a sheep on Farmville!”). Apps incentivized players to grant the app access to their friends’ information by restricting play unless they did so, or by letting the players score higher in the game if they did. But the problem went much deeper than annoyance.

When a player granted an app permission to access their friends, it was all or nothing: The app didn’t just get access to a list of people; it could see everything the player could see — friends’ birthdays, phone numbers, posts, check-ins, groups, and more.

Facebook likes to emphasize the role of user choice in control of privacy settings, but app users may not have realized what they were agreeing to when granting access, and app users’ friends were, of course, not consulted at all.

As a computer scientist, I knew it was foolish to grant that kind of access to dubious quizzes, games, and the like, but I watched in exasperation as my friends eagerly installed them, revealing my information without my consent. I even considered unfriending certain especially app-promiscuous friends, but, realizing this task was Sisyphean, I resigned myself to unwanted and unapproved data leaks.

Facebook might retort that even friends of app users retained some control over their data: If they opted not to let their friends see their birthday or phone number, or even status updates, apps couldn’t see those things either! But many users are on Facebook because they enjoy small pleasures like having their friends send them birthday wishes. Should they have to choose between that pleasure and losing their privacy to third-party developers?

And what is the point of updating a Facebook status if your friends are blocked from seeing it? Yet this was the “choice” that privacy-minded Facebook users confronted at that time.

Since then — in 2014, to be specific — Facebook has decoupled some of these privacy settings: Now you can show your friends your birthday but hide it from your friends’ apps. The screenshot below shows what the controls look like today. I’ve turned off all access, but the default setting is that every piece of data you see listed here is something that your friends’ apps learn about you:

Screenshot by Cynthia Lee

Whether or not you’re comfortable with Facebook having all this data about you, you should definitely not trust third-party app developers with it. Facebook does a cursory check of apps to make sure they meet requirements like not crashing, but there is essentially no vetting of the individuals behind the apps to ensure that they are trustworthy, or even know what they’re doing.

App owners agree to terms of service that prohibit them from, for example, taking every “friends only” post you’ve ever written and posting it publicly. But the apps have this data, and in practice, Facebook has no way to ensure they don’t abuse it. Facebook doesn’t even have a good way to punish violators after the fact, except to ban their apps from collecting new data on Facebook users in the future.

Once the app-makers have the data, Facebook doesn’t have an effective way to get it back. Imagine that 30 years from now an amateur author of an obscure, long-defunct Facebook third-party app dusts off their server and remembers they still have the private data of a million people. Now imagine they use the names and phone numbers — along with years of recorded photos and memories — to defraud now-elderly people by convincingly pretending to be a long-lost acquaintance.

Zuckerberg told senators last week, “It’s not enough to give people control of their information, we have to make sure developers they’ve given it to are protecting it too.” So far, he hasn’t come close to achieving either goal.

Facebook tracks you even if you’re not on Facebook

Over the past few weeks, there have been many stories about people who have decided to reduce their activity on Facebook, or even delete their account altogether. But this is not enough to ensure that Facebook doesn’t continue to collect personal data about you. Here are some examples of ways Facebook gathers data on users — and non-users — that are less obvious than just warehousing what you write and do on the site:

  • Off-Facebook browsing data: Through use of web browser cookies and tracking “pixels” — tiny images, too small to see, tucked away in a corner of many webpages — Facebook can track your browsing habits even when you are on websites that are not owned or operated by Facebook. For example,, Huffington Post, Drudge Report, and, yes, Vox all have embedded pixels or social sharing buttons that track which articles you read. This provides websites and their advertisers critical information about audience demographics, size, and whether the ads lead to purchases, but it also provides Facebook with this tracking data.
  • Non-users’ browser data: Facebook may not know the names of non-users, but using pixels and browser tracking, they can still accumulate a demographic profile based on their use of other websites — like news and shopping — that Facebook sees their device repeatedly visit. Facebook can then suggest ads that appear on those other sites that are tailored to the person’s interests, all without that person ever having visited Facebook or created an account.
  • Non-users’ friend networks: Facebook not only knows what non-users read and buy online, but the company may also have a pretty good idea who their friends are. That’s because someone using Facebook’s “Find Friends” feature uploads all their contacts, without those friends’ permission. (This is not unlike the pre-2014 app problem.) Each time someone who isn’t on Facebook is listed among those contacts, Facebook learns more about their social and business ties.
  • Facial recognition: If your face has appeared in group photos that have been posted to Facebook, the website learns to recognize you with biometric facial analysis. Facebook can then track who you spend time with by analyzing photos, whether or not you’ve been tagged in them. And if those photos have GPS tags attached, they’ll have your location data as well.

Given the near monopoly Facebook enjoys on what is now a key piece of infrastructure in our global society, protecting our data is essential. I’ve heard many friends tell me that recent revelations about privacy problems have made them feel ashamed of their social media use, or ashamed that they find the idea of quitting to be overwhelmingly difficult, or ashamed that their attempt at giving up Facebook for Lent fell apart only a few days in.

I don’t think anyone should feel ashamed of wanting to be connected to their friends and family. A better answer than disconnecting from each other is to ensure that those who connect us do so safely and responsibly.

Cynthia Lee is a lecturer in the computer science department at Stanford. She founded the website Peer Instruction for Computer Science to support educators in flipping their computer science classrooms using peer instruction. She has a PhD in high-performance computing.

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