I'm going to assume you're using mod_auth_cas v1.1.
Are you using a /samlValidate endpoint? Something like:
CASValidateURL
https://login.example.org/cas/samlValidate
CASValidateSAML On
If so and you're not getting attributes, check with your CAS server admin.
If you aren't using /samlValidate, the current version of mod_auth_cas
does not support CASv2 attributes with /serviceValidate.
You have 2 options:
1. Use /samlValidate.
2. Try this merge request:
https://github.com/Jasig/mod_auth_cas/pull/110. I've successfully
tested it and it should be merged soon.
As for your require statement, you probably want something like:
# assuming Apache 2.4
# be sure to replace GROUP_ATTRIBUTE!
require cas-attribute GROUP_ATTRIBUTE:ADMIN