Today I was getting a Problem in ubuntu that is
sudo: /etc/sudoers is mode 0777, should be 0440
sudo: no valid sudoers sources found, quitting
I solved it in following way :-
I have put Ubuntu onto a LiveCD, boot into the LiveCD, mounted my Ubuntu Partition on the Hard Drive (Open a file browser, like Nautilus, and click your Ubuntu Partition; or click Places->"xxGB Partition/Your Ubuntu Partition").
Then, opened a terminal within the LiveCD session and type these two commands:
sudo chmod 0440 sudoers
Thats it :)