> We have our Shibboleth IDP using CAS as the only login handler resulting in 
> CAS being the manager of the SSO session and Shibboleth being simply a 
> pasthrough for SAML.  Since the Shibboleth IDP does not maintain an SSO 
> session it should redirect to CAS for each auth request  to get a new Service 
> Ticket.

While your expectation is correct, I have the feeling you want to
prompt the user for credentials on every federated service access
request. Unfortunately that is not possible.

If you disable the Shib "Previous Session" authn handler [1], which is
the meaning I take from "Shibboleth IDP does not maintain an SSO
session", you will still incur exactly one CAS authentication. The
reason is that for any browser-mediated SAML profile, browser cookies
come into play. Here's the flow:

1. User browses to spA.example.org
2. User selects home institution and is redirected to idp.bar.edu
3. IdP engages CAS authentication provider, which triggers redirect to
CAS, cas.bar.edu
4. No CASTGC cookie is present, so user is prompted to login to CAS
5. User authenticates to CAS and is redirected to idp.bar.edu with ticket
6. IdP validates ticket and authn engine completes successfully
7. IdP redirects to spA.example.org with SAML response
8. ... time passes ...
9. User browses to spB.example.org
10. IdP engages CAS authentication provider, which triggers redirect
to CAS, cas.bar.edu
10. CASTGC cookie is present so CAS issues ticket to IdP via redirect
11. Steps 6-7 as before

So while a new ST is granted for every federated service access
request, only one login occurs, the one that initiated the CAS SSO
session. Thus you will see exactly one CAS login for a number of
federated service access requests that happens during the lifetime of
the CAS SSO session. While the renew CAS protocol parameter could
theoretically help you here, the SP would have to set that flag, which
is outside of your control.

I hope that explains the behavior you are seeing.

[1] https://wiki.shibboleth.net/confluence/display/SHIB2/IdPAuthPreviousSession

M

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