Mechanisms to use network session identifiers for software-as-a-service authentication

Techniques are provided for authenticating a subject of a client device to access a software-as-a-service (SaaS) server. A network access device receives a request from a client device to establish a network session and transfers identity information of the subject, the client device and the network session to a session directory database. A request is sent to access an application on a SaaS server. If it does not contain an identity assertion that identifies the subject, the request is redirected to an identity provider device, to provide identity assertion services to the subject. A network session identifier is inserted into the request by a network access device and the request is forwarded to the identity provider device. The identity provider device uses the network session identifier to query the session directory database for the identity information to be used for a security assertion of the subject to the SaaS server.