Anything can be hacked given enough time and motivation.
In the case of hijacking a session, a hacker would need to intercept the HTTP Response from the web server, parse out the session cookie, and then generate a HTTP Request based on the cookie.
So it is still possible, but just getting the cookie doesn't give you access to the contents of the session variables. Those are stored on the web server, not in the cookie. All it allows you to do is represent yourself to the web server as if you are the client for whom the session was created. So the hacker has no more access to information than the user whose session has been hijacked.