Abstract: The invention relates to a mechanism and a coupling device, a so-called secure switch, for securing data access of a first subscriber to a second subscriber, wherein the secure switch has a port that is configured to provide an endpoint of a tunnel to the second subscriber through which data can be securely transmitted via an insecure network. The tunnel is established in the secure switch in place of the downstream first subscriber. The invention is advantageous in that security functions can be integrated into existing networks at a later point in time by inserting secure switches.