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Right now there is no support for Central Authentication Service in XOOPS.
There is an official
phpCAS implementation, so it would be a matter of adding an authentication method that spoke CAS. It could probably leverage the existing provisioning mechanism.
If someone wanted to write this adaptation and contribute it, that would be great. While it sounds like a good enhancement, it is not likely to rise to the top of the list right now due to limited resources.
If the underlying authentication is AD, you could still have common authentication credentials, but not a true SSO.
This does sound like an interesting endeavor, and I wish you good luck!