I just happened to be unable to log in to my Mac using ScreenSharing (VNC). It is enabled, but probably the firewall is asking if ScreenSharing should be allowed to receive incoming connections… Bummer!
If at least SSH is working, it is possible to disable the firewall completely from the command line. Please be sure to only do this in a secured environment:
sudo defaults write /Library/Preferences/com.apple.alf globalstate -int 0
After this you will have to restart the firewall agent:
sudo launchctl unload /System/Library/LaunchDaemons/com.apple.alf.agent.plist sudo launchctl load /System/Library/LaunchDaemons/com.apple.alf.agent.plist
You should now be able to use ScreenSharing.
Do not forget to re-enable the firewall again after you have finished your work, same procedure, but with 1 instead of 0, of course 🙂
To enable ScreenSharing in general, please read this older post: Activate screen sharing on Mac OS X when you only have SSH enabled