How to maintain state parameter in Identity Provider (IdP) initiated SAML sso?

Question

I started with Service Provider based SSO for SAML. Since the user had to enter his email before proceeding with the login, a state variable was initiated and passed on to the SSO. It comes back through the callback URL and hence was check again for the sanity purpose. It protected against CSRF attacks.

Now IdP initiated SSO doesn't allow me to set state variable at all. Login starts at Identity Provider and only an auth token is provided to the app. I do not know which user is authenticating from the beginning. If I remove the state variable check, it could trigger a CSRF attack as well.

I am also using omniauth in rails which makes state variable a compulsory param and SSO provider is auth0 .

What is the way to attach state variable to IdP initiated SSO solutions?

Answer 1

The de-facto standard value of the RelayState parameter in IDP-init-SSO SAML flows is the URL that you want to send the user to after successful validation of the SAML assertion at the SP. That would work for the vast majority of SAML deployments out there.

However, that mechanism indeed does not protect against CSRF attacks this is why the spec is silent on the value of RelayState in IDP-init-SSO and leaves it open to agree on mechanisms between IDP and SP to prevent CSRF through the value of that parameter. One such mechanism would be to use a signed value in the RelayState but as said, nothing is standardized and it would thus depend on a bi-lateral agreement between IDP and SP which does not scale.

In summary: send the value of the URL that you want the user to go to as the RelayState value in the "unsolicited" SAML Response that you send to the SP. How you get the IDP's SAML stack to do that is implementation specific. For Auth0 you can read on this at: https://auth0.com/docs/protocols/saml/saml-configuration#idp-initiated-sso , and in your case it would look like: https://{accountname}.auth0.com/samlp/YOUR_CLIENT_ID?RelayState=http://FINAL_DESTINATION_URL