Services like Twitter and Instagram offer private accounts. Labeled with a comforting digital padlock, these accounts claim to protect your data from the public – only people you approve can see your information. The digital lock is comforting, but how much does it mean? How private is the data in a private account? Less private than you may think.

The weakness of private profiles became apparent to me a couple of weeks ago when I received an email from Instagram with a photo from a private account that I had not even requested to follow. To clarify: Instagram sent me a photo of someone’s family from a private account, whose owner had never approved me to see it.


Your first reaction might be something like “What are those idiots at Instagram up to!” But I assure you, they are not incompetent, nor are they nefarious. They are simply neglecting the importance of information flow (explained below) in privacy.

Using this bug as a motivating example1, I will explain why information flow control is so important in guaranteeing user privacy, why it is ignored, and what we can do about it.

The Bug

Recently, I deleted the Instagram app from my phone. Only a few days later, Instagram sent me an email trying to get me to return to using their service by sending me an email with photos from my friends. The email also included photos from suggested friends, one of whom had a private account. We’ll call him Bob.

Here are the important things about how the email dealt with Bob’s information:

  • The email itself included Bob’s profile picture and one image from his account, in the same format as accounts that I followed.
  • When I clicked on his account name, I was brought to a page that said “This account is private.” This page showed his profile picture and account name, but not any of his photos.
  • When I clicked on the image itself, I was led to a “Sorry, this page isn’t available” page.

Instagram did a good job hiding Bob’s private images when I tried to directly request them but the email itself still contained the image. Why is Instagram able to block me from seeing such an image, but not able to stop it from being emailed to me?

The answer lies in the difference between access control and information flow control.

Access Control and Information Flow Control

Both access control and information flow control describe mechanisms for protecting “private” information, information that should be visible to some parties but invisible to others.

What makes information flow tough?

All of these social media services do a good job with access control - if I click on private profile in the app, I won’t be able to see any of its posts. But, that is only one way to see those posts. When companies start adding on extra services - friend recommendations, email digests, top stories - they often fail to maintain the expected level of privacy.

How do we fix it?

Relying on one programmer, let alone all of them, to write bug-free code is unreasonable. We need a more systematic solution that will reduce the small human errors that create unintended information flows. One way to do this is to use the type system of a programming language to provide strong guarantees about the flow of sensitive data.

JIF by Andrew Myers is an example of a language extension that uses “labels” to restrict where sensitive data can flow. Jeeves by Jean Yang2 is another solution that makes it easier to enforce information flow policies, through a technique called “policy-agnostic programming.”


  1. Only a couple of weeks later, another information flow bug led a reporter to finding FBI Director Comey’s Instagram account.
  2. Disclaimer: Jean is the reason I am interested in information flow and encouraged me to write this post :)