The technology disclosed relates to non-intrusively enforcing security during federated single sign-on (SSO) authentication without modifying a trust relationship between a service provider (SP) and an identity provider (IDP). In particular, it relates to configuring the IDP to use a proxy-URL for forwarding an assertion generated when a user logs into the SP, in place of an assertion consumer service (ACS)-URL of the SP. It also relates to configuring an assertion proxy, at the proxy-URL, to use the SP's ACS-URL for forwarding the assertion to the SP. It further relates to inserting the assertion proxy in between the user's client and an ACS of the SP by forwarding the assertion to the SP's ACS-URL to establish a federated SSO authenticated session through the inserted assertion proxy.