An authentication request is generated when a user of a client device attempts to initiate a user session with an application managed by a service provider. An authentication response is generated based on credentials received from the user. The authentication response includes an assertion on behalf of the user. A delivery resource locator for the assertion is rewritten to a resource locator of a proxy in order to redirect the assertion to the proxy. The authentication response is sent to the client device together with the resource locator of the proxy in order to cause the client device to send the assertion to the proxy that decodes the re-written resource locator and sends the assertion to the service provider.