source: http://www.securityfocus.com/bid/1513/info There is a vulnerability in the Linux pam_console module that could allow an attacker to remotely reboot the workstation or perform other actions limited to local users. If a workstation is configured to use a display manager (xdm, gdm, kdm, etc.) AND has XDMCP enabled, it is possible for a user who logs in remotely to use Xnest -query to log in on display :1, which is recognized as the system console. This vulnerability is only present if the workstation is running a graphical login manager such as gdm or kdm. This description of how to replicate the problem was posted to RedHat's Bugzilla bug-tracking system by firstname.lastname@example.org: 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 <email@example.com>: 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.