In this scenario, the SP sends an HTTP redirect message to the IdP containing an authentication request. The IdP returns a SAML response with an assertion to the SP via HTTP POST.

SP-initiated SSO: redirect/POST

Processing steps:

  1. A user requests access to a protected SP resource. The user is not logged on to the site. The request is redirected to the federation server to handle authentication.
  2. The SP returns an HTTP redirect (code 302 or 303) containing a SAML request for authentication through the user's browser to the IdP's SSO service.
  3. If the user is not already logged on to the IdP site or if re-authentication is required, the IdP asks for credentials (for example, ID and password) and the user logs on.
  4. Additional information about the user may be retrieved from the user datastore for inclusion in the SAML response. (These attributes are predetermined as part of the federation agreement between the IdP and the SP—see User attributes.)
  5. The IdP's SSO service returns an HTML form to the browser with a SAML response containing the authentication assertion and any additional attributes. The browser automatically posts the HTML form back to the SP.

    SAML specifications require that POST responses be digitally signed.

  6. (Not shown) If the signature and the assertion (or the JSON Web Token) are valid, the SP establishes a session for the user and redirects the browser to the target resource.