* Cause and symptoms
sudo breaks when one or more of the following occurs:

  • The /etc/sudoers file has been altered to no longer allow users in the admin group to escalate privilege
  • The permissions on the /etc/sudoers file are changed to something other than 0440
  • A user who should not have been has been taken out of the admin group

Basically, the solution is to change ownership back to root. Culling bits and pieces from different sources, here’s what I ended up doing:

  1. Rebooted and hit ESC at the prompt to get the grub menu.
  2. Choose “…(Recovery Mode)” from the grub menu.
  3. Choose “Drop to root shell prompt
  4. Ran the following commands:
    chown root:root /usr/bin/sudo
    chmod 4755 /usr/bin/sudo
    chmod 0440 /etc/sudoers
    reboot

When the system came back up, everything was normal again.

 

http://kotowanandesu.blogspot.com/2010/06/sudomust-be-setuid-root.html