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Any user is able to execute /usr/bin/shutdown now (soft link to /usr/bin/consolehelper) without -h or -r parameters... So, when an ordinary user executes: /usr/bin/shutdown now password: <user's password> The system goes down to runlevel 1 (single-user)... This "bug" can only be exploited on local machine, but, when used against a remote red hat server, it takes the machine down until any admin restarts it...
I guess RedHat should adjust pam and usermode packages to fix it, because it is necessary to remove this "bug" on each machine "available" to public use... Sorry for "yet" another message about this problem (I have seen posts about usermode bugs just a second after I submitted my text :-), but I think it is a HUGE security hole it should be fixed... It is "really" annoying when you discover a lot of local user on a fresh RedHat installation :-)
A user with access to the console (which is required to do this) can also just switch the computer off. That said, this is configurable using PAM's configuration file mechanism. Removing the files named "shutdown", "halt", "reboot", and "poweroff" from the /etc/pam.d directory will disable this.