Wordpress has been CASified via an extension, as documented here http://wordpress.org/extend/plugins/cas-authentication/
A Wordpress CAS Authentication plugin WP Cassify is available here : https://fr.wordpress.org/plugins/wp-cassify/. This is a highly configurable plugin. It's compatible with the latest version of Wordpress and tested up with CAS version 4.1.4. You can enable plugin by blog or on network for multisite installs.
The online documentation is available here : https://wpcassify.wordpress.com/.
A WordPress Multi-User modification (not extension) to CASify it, is documented here: http://www.andrejciho.com/wordpress-mu/wpmu-cas-integration/