Facts

Any Mac app, sandboxed or not sandboxed can:

  • Take screenshots of your Mac silently without you knowing
  • Access every pixel, even if the Mac app is in the background
  • Use basic OCR software to read the text on the screen
  • Access all connected monitors

What’s the worst that could happen?

  • Read password and keys from password managers
  • Detect what web services you use (e.g. email provider)
  • Read all emails and messages you open on your Mac
  • When a developer is targeted, this allows the attacker to potentially access sensitive source code, API keys or similar data
  • Learn personal information about the user, like their bank details, salary, address, etc.

Disclaimer

This project is a proof of concept and should not be used in production. The goal is to highlight a privacy loophole that can be abused by Mac apps.

How can I protect myself as a user?

To my knowledge there is no way to protect yourself as of now.

Proposal

There are lots of valid use-cases for Mac apps to record the screen, e.g. 1Password 2fA support, screen recording software or even simple screen sharing via your web browser or Skype. However there must be some kind of control:

  • The App Store review process could verify the Sandbox entitlements for accessing the screen
  • Put the user in charge with a permission dialog
  • Additionally the user should be notified whenever an app accesses the screen.

Of course, I also filed a radar (rdar://37423927) to notify Apple about this issue.

How does it work?

A developer just needs to use CGWindowListCreateImage to generate a capture of the complete screen within an instant:

CGImageRef screenshot = CGWindowListCreateImage(
  CGRectInfinite, 
  kCGWindowListOptionOnScreenOnly, 
  kCGNullWindowID, 
  kCGWindowImageDefault);

NSBitmapImageRep *bitmapRep = [[NSBitmapImageRep alloc] initWithCGImage:screenshot];

In my experiments, I piped the generated image over to a OCR library and was able to get all text that was rendered on the user’s machine.

Unless otherwise mentioned in the post, those projects are side projects which I work on on weekends and evenings, and are not affiliated with my work or employer.