If the user has a web-socket-server running (for example because he is a developer) could the page read from it?
Only if that server chooses to accept the request, which it can decide based on the Origin header.
Personally, I was more concerned with getting spurious requests on ports bound to 127.0.0.1 (which I've been using for IPC), but that issue already existed before the introduction of WebSockets.
WebSockets of course do make things like port scanning easier, but as others have pointed out, you could already do that with a bit of ingenuity eg through tracking response times.
Only if that server chooses to accept the request, which it can decide based on the Origin header.
Personally, I was more concerned with getting spurious requests on ports bound to 127.0.0.1 (which I've been using for IPC), but that issue already existed before the introduction of WebSockets.
WebSockets of course do make things like port scanning easier, but as others have pointed out, you could already do that with a bit of ingenuity eg through tracking response times.