Facts

Any Mac app, sandboxed or not sandboxed can:

• Take screenshots of your Mac silently without you knowning
• 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 or screen recording software, 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.