CentOS 6 is now EOL (End of life).
Unfortunately there's a nasty little bug in the sudo application that can cause non-privileged users to gain root access.
To patch it, download the RPM directly from the sudo site and upgrade:
rpm -Uvh https://github.com/sudo-project/sudo/releases/download/SUDO_1_9_5p2/sudo-1.9.5-3.el6.x86_64.rpm
If you run sudo -V you should now see:
Sudoers I/O plugin version 1.9.5p2