SAML implementations can protect against this by turning IdP-initiated requests into an SP-initiated request. On the SP-initiated responses, there is an `InResponseTo` field that can be validated (or `RelayState` can be made to work).
The OIDC specification is pretty hand-wavy about how to prevent this attack as well. So don't assume all OIDC implementations do.
The OIDC specification is pretty hand-wavy about how to prevent this attack as well. So don't assume all OIDC implementations do.