Via the CLI, obtain the number of active sessions is exposed. e.g.

[standalone@embedded /] /deployment=foo.war/subsystem=undertow:read-attribute(name=active-sessions)
{
    "outcome" => "success",
    "result" => 0
}

Programmatically, you can call undertow/SessionManager.java at master · undertow-io/undertow · GitHub

You can obtain a reference to the Undertow SessionManager via the UndertowDeploymentService which has a getDeployment() method which returns an Undertow Deployment, from which you can call getSessionManager().getActiveSessions();

What you asking for is the equivalent of HttpSession.getSessionContext().getId(), which was deprecated in Servlet 2.1 for security reasons.

Thus, you can achieve this programmatically through the use of internal Undertow classes, following the instructions above.

write a ServletExtension http://undertow.io/undertow-docs/undertow-docs-2.0.0/index.html#servlet-extensions