A VPN gateway is described that provides single sign-on (SSO) functionality with respect to remote users who have established tunneling sessions with the VPN gateway and who attempt to access a protected resource. The VPN gateway may receive, from a client device, a security assertion request that includes a request for a security assertion to be made by the VPN gateway with respect to a user of a private network associated with the VPN gateway, determine whether the security assertion request was received via a tunneling session established for the user between the client device and the VPN gateway, and issue a security assertion for the user in response to determining that the security assertion request was received via the tunneling session. In this way, a VPN gateway may act as an SSO identity provider for users that have an established tunneling session with the gateway.