I’ve just helped a colleague with a problem (beyond the mental one), when he tried to execute sudo. For some reason, EVERYTHING in his /etc/ had 777 permission - that’s why the sudo: /etc/sudoers.d is world writable. I tried to login as root, executing su, but su: Authentication failure was shown… how could I change the permissions if I wasn’t able to login as root? Follow the instructions above to solve this problem.

To login as root, without su or sudo, you can use pkexec:

pkexec su

Now change the files’ permissions:

chmod 440 /etc/sudoers
chmod 775 /etc/sudoers.d
chmod 440 /etc/sudoers.d/README

That’s all - now you must be able to sudo.