The following errata for CentOS-2 have been built and uploaded to the centos mirror:

RHSA-2005:799-01 Moderate: ruby security update

Files available:
irb-1.6.4-2.AS21.2.i386.rpm
ruby-1.6.4-2.AS21.2.i386.rpm
ruby-devel-1.6.4-2.AS21.2.i386.rpm
ruby-docs-1.6.4-2.AS21.2.i386.rpm
ruby-libs-1.6.4-2.AS21.2.i386.rpm
ruby-tcltk-1.6.4-2.AS21.2.i386.rpm

More details are available from the RedHat web site at
https://rhn.redhat.com/errata/rh21as-errata.html

The easy way to make sure you are up to date with all the latest patches is to run:
# yum update