The Supermicro BMC implementation allows remote attackers to bypass authentication and execute arbitrary IPMI commands by using cipher suite 0 (aka cipher zero) and an arbitrary password.
https://lists.gnu.org/archive/html/freeipmi-devel/2013-02/msg00013.html
http://www.wired.com/threatlevel/2013/07/ipmi/
http://www.metasploit.com/modules/auxiliary/scanner/ipmi/ipmi_cipher_zero