Multiple Linux Vendor pam_console Remote User Vulnerability
This description of how to replicate the problem was posted to RedHat's Bugzilla bug-tracking system by email@example.com:
1. ssh into the server, allowing ssh to establish a secure forwarded X connection. If no one else is using display number 0, you'll end up with a DISPLAY value of "host:0.0".
2. Invoke "Xnest -query localhost" on the remote machine.
3. Log in, starting a Gnome session.
4. From the Gnome panel, select "logout". You'll be presented with the option of shutting down or rebooting the server.
Another description, this one from Andreas Hasenack <firstname.lastname@example.org>:
1. login remotely (X -broadcast) (have gdm, kdm, whatever running with XDMCP enabled somewhere)
2. after login, start Xnest with -broadcast again, for example
3. login again, now you will be using display :1
4. this is treated as a console user, and commands only available to console users can be run, such as reboot.