Yum-cron

yum-cron will work just like Debian's unattended upgrades. Use it to keep your RedHat or CentOS* host up-to-date with security patches and even regular package updates. yum-cron is an alternate interface to yum, optimized for cron.

update_cmd = default is the only option that works "out of the box" for CentOS -- meaning you can't be choosy and can't apply only security updates. The work-arounds are: use a different distro (e.g. Scientific Linux does security updates automatically), or become a patron of Steve Meier's repository which adds in the security metadata

Similarly, you can use yum autoremove to remove obsolete packages from your system.