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linuxmint / cinnamon-screensaver

The Cinnamon screen locker and screensaver program
GNU General Public License v2.0
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Security: Lockscreen bypass when resizing screen #460

Open algorithmwolf opened 1 week ago

algorithmwolf commented 1 week ago

Distribution

Mint 22

Package version

6.2.0

Graphics hardware in use

Radeon 680M

Frequency

Always

Bug description

Cinnamon temporarily shows the desktop and allows interacting with it when the screen is resized.

This was tested using NoMachine: https://www.nomachine.com/ which effectively plugs into the real display that outputs to the HDMI port, unlike other remote desktop solutions that create virtual displays etc

In such a situation, a malicious user could potentially kill the cinnamon-screensaver process giving them full access to the desktop without knowing the password.

Steps to reproduce

  1. Lock display
  2. Resize desktop by enabling remote resizing in NoMachine and then maximizing or resizing the NoMachine window
  3. Desktop with all your open apps and information is visible for a few miliseconds. At this point, you can see everything that was on the desktop, and keystrokes are being passed to programs
  4. Desktop is locked again shortly after

Expected behavior

Instead of the desktop being revealed temporarily during screen resizing, the lockscreen should remain in place and block any access to the desktop.

Alternatively, it could be implemented as a black screen that captures all input while the locker restarts after the resizing operation finishes.

Additional information

I recorded this behaviour in a video: https://github.com/user-attachments/assets/87f195aa-6f96-4798-8a9f-67c278560ddd

The cinnamon-screensaver logs: cinnamon-screensaver.log

algorithmwolf commented 1 week ago

Seems related to #239

My (un?)educated guess of what's happening here is the actual cinnamon-screensaver process is crashing, and a helper process is immediately spawning a new process upon detecting this crash, because this helper process knows the system was supposed to be locked.

My suggestion is that this helper process should additionally draw a blank screen completely blocking the desktop right behind the lockscreen, in case the lockscreen ever crashes.

algorithmwolf commented 1 week ago

Also looks related to #440 and #437