VULNERABILITY DETAILS
Exploit I found on how to get Guest mode on a managed Chromebook with no Guest Mode permission.

VERSION
Chrome Version: [65.0.3325.209 (Official Build) (64-bit)] + [stable]
Operating System: Chrome OS, 10323.67.0 (Official Build) stable-channel candy

REPRODUCTION CASE
Press Esc+Refresh+Power (Recovery) and then Ctrl+D, after that press space, then enter. That should enable verified mode again. once it reboots to the setup screen connect to WiFi, it should say something about sending usage data to Google. Click accept, right when the loading screen comes up go to the bottom right of the taskbar and disable WiFi. It will keep loading for a minute or so then a screen will pop up asking you to connect to a new network or "Use Guest Mode". Select use guest mode and it works, no policies and Crosh.

Hope I helped, I'm just trying to make some money.

Sincerely,
Sean Kahler
support@shopbattledash.com