Consider the steps involved in WS-Federation Passive Requestor Profile1:
The above figure shows a sequence diagram of a user (requestor) accessing a web application with his browser. Since the user was not authentication due to a recent session, the application redirects the user to the IDP for a user login (1). The IDP collects the credentials from the user and uses a Security Token Service (STS) to validate the credentials and also to get a SAML token from the STS (2). The STS itself is connected to a LDAP data store to validate the user credentials and also to retrieve additional information (claims) about the user, e.g. roles. On successful authentication (3) the IDP returns the SAML token issued by the STS (4) to the user and advices (auto-submitting form) the user to send this SAML token to the originally requested web application (5). The IDP takes care of providing a web user interface and handling URL redirects, whereas the STS is responsible for generating SAML Token and validating of user credentials. The web application validates the SAML token (6) and on success returns the desired web page (7).
In step 4, we can return a form to the user that automatically submits itself. Does this form require any CSRF protection on it?
I am looking for confirmation, but I think the answer is "no". Even if a malicious website could cause a user to POST this form, it would also have to successfully forge a security token.