You should 'yum update' as soon as possible to resolve this issue.
Here's why you should care:
https://securityblog.redhat.com/2014/09/24/bash-specially-crafted-environmen...
Links to the centos updates:
CentOS-5: http://lists.centos.org/pipermail/centos-announce/2014-September/020582.html
CentOS-6: http://lists.centos.org/pipermail/centos-announce/2014-September/020585.html
CentOS-7: http://lists.centos.org/pipermail/centos-announce/2014-September/020583.html