The patch for CESA-2017:2563 announced at https://lists.centos.org/pipermail/centos-announce/2017-August/022529.html is only available in the centos 6.9 repo. It is missing for the generic centos-6 repo at http://mirror.centos.org/centos-6/6/os/x86_64/Packages/. Typically this repo contains all the latest patches for the latest minor release, but it's missing the new version of openssh. I assume this is by mistake, but please correct me if I am wrong.
Thanks Dustin