Woops!
I just installed x11vnc and tightvnc-java. I set up a cool .x11vncrc in my home directory to keep the vnc session open, to look for a password I had generated with x11vnc -storepasswd, and so on. Pretty darn useful to connect to an already extant X desktop (like winders VNC) rather than creating some new one from scratch. I typed `x11vnc` and it started up according to my .x11vncrc configuration, then I connect to my computer from work --

I then realized that the X desktop it hooked me up to was not my own. It was another user I had created and left logged in to X. I never had to enter that user's password; it only checked the password in the password file I told it to generate in one of my directories. So, on a system where x11vnc is installed, could you not gain access to any other user's running X desktop? How could I make this more secure?

in order for the users desktop to be made accessible without using a password then they will have needed to run x11vnc with options to not require one. i don't see a security hole other than stupid users.

But that other user did not run x11vnc, I did. In other words, x11vnc just hooked onto display :0, regardless that it was not my X desktop! That user whose X desktop I can have access to may be stupid, they may be smart, who can say? There was nothing that other user did to configure anything one way or another, all defaults so far as I know. Should some user, me, be able to access their desktop via x11vnc? Whether or not I "stupidly" ran x11vnc isn't going to make the other user feel better when I steal all their embarassing holiday photos or send incriminating emails, etc etc etc.

i'm not a position to test this but I would assume that your default xhosts configuration is less than ideal. by deafult only the server owner on local host should be able to connect to the server instance. if you run "xhost +localhost" then that would explicitly allow any local user to connec to the server, and allow things like x11vnc to be run. what is configured for that user if you run "xhost"?

If this is the default behavior, isn't that pretty unsafe? I wouldn't know to change anything to keep someone gaining access to my desktop through VNC. I only noticed this by accident...
If I run `xhost` I get: