I configured spring-security-cas in my webapp for generating proxy tickets.
During ticket validation we provide a pgtUrl parameter and then the CAS server tries
to connect to our webapp (using the configured proxyReceptorUrl) to determine is this
URL is a valid end-point but it result in a 404 because the request is passed trough
the servlet but is not handled (because no controller is mapped for this URL).
If we set the grails.plugins.springsecurity.rejectIfNoRule parameter  to true or if I mark this
proxy receptor URL with the role "IS_AUTHENTICATED_REMEMBERED" then it works:
the ping request  result in a 302 (redirection to CAS because the request needs to be authenticated)
the pgtId/pgtIou request result also in a 302 (therefore without the proper XML fragment) but the mapping is stored
This works because the CAS server is not strict about the expected XML fragment,
only the a proper status code is mandatory .
Current workaround is to create a controller for this URL with anonymous access and
always respond with a 200 and some text.
I believe this issue is not related to Grails and may impact any webapp using
spring-security-cas and wanting to generate proxy tickets.
Furthermore, the default behaviour in cas-client  is already to stop processing the
request once it has been handled by the Cas20ProxyReceivingTicketValidationFilter .
Thank you for your bug/patch submission. I fixed this is 3.x and master, but went about it slightly differently. In short, the fix moves CommonUtils.readAndRespondToProxyReceptorRequest into CasAuthenticationFilter.attemptAuthentication. This makes sense since the CAS server is authenticating that the proxy url is valid (i.e. it exists and the SSL handshake succeeds). It also allows the FilterChain to not be processed by returning a null Authentication.