The embedded HTTP server does not invalidate the previous session identifier when an application changes the session id, e.g. after login. An attacker who can plant a session identifier in a victim's browser can hijack the victim's authenticated session. Users are recommended to block network access to the embedded HTTP server from untrusted clients, or to migrate to the Tomcat based TomEE distributions.
Session Fixation Authenticating a user, or otherwise establishing a new user session, without invalidating any existing session identifier gives an attacker the opportunity to steal authenticated sessions.