Some times you can not use the YUM to install mod_security , in these situation, try this:

nano /etc/yum.repos.d/utterramblings.repo

and set these in the file:

name=Jason’s Utter Ramblings Repo
baseurl=http://www.jasonlitka.com/media/EL$releasever/$basearch/
enabled=1
gpgcheck=1
gpgkey=http://www.jasonlitka.com/media/RPM-GPG-KEY-jlitka

and by Ctrl X and Yes save the file.

Now you can simply install the mod_security:

yum install mod_security