If you want to be sure all security updates are installed automatic you can use yum-cron. Be sure you have configured your yum correctly not to update wrong things from different repositories.
- Install yum-cron
yum install yum-cron
- add yum-cron to your runlevels
chkconfig --levels 35 yum-cron on
- start yum-cron
service yum-cron start
That's it. Check /var/log/yum.log frequently for errors and other information.