It is easy enough to 'fetch' (ie draw locally) a remotely running linux application: If I `ssh -Y` to the remote machine and run an application, that application will surely enough pop up in my local desktop.

If , however, while I am ssh'ed in the remote machine, I `su` to a different user, I can not forward the X application to my local machine. It says `wrong authentication`. 

I am not sure how to tackle this case. The `echo $DISPLAY` is still correct (set by the initial `ssh -Y` logon), but the session cookie is probably only set for the initial user that logged on ssh.

How can I overcome this difficulty and forward other X applications that are run from different users?

The reason I am not ssh'ing directly is because I don't want that user to be accessible through ssh (it is the "virtualbox" user, which is obviously an easy target for bots trying to ssh to that server)...