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

RHSA-2005:381-01 Low: nasm security update

Files available:
nasm-0.98-8.EL21.i386.rpm
nasm-doc-0.98-8.EL21.i386.rpm
nasm-rdoff-0.98-8.EL21.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