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Apart from the actual CPU, modern server motherboards contain other auxiliarycomponents, for example voltage regulators for power management. Those areconnected to the CPU and the separate Baseboard Management Controller (BMC) viathe I2C-based PMBus.  In this paper, using the case study of the widely used Supermicro X11SSLmotherboard, we show how remotely exploitable software weaknesses in the BMC(or other processors with PMBus access) can be used to access the PMBus andthen perform hardware-based fault injection attacks on the main CPU. Theunderlying weaknesses include insecure firmware encryption and signingmechanisms, a lack of authentication for the firmware upgrade process and theIPMI KCS control interface, as well as the motherboard design (with the PMBusconnected to the BMC and SMBus by default).  First, we show that undervolting through the PMBus allows breaking theintegrity guarantees of SGX enclaves, bypassing Intel's countermeasures againstprevious undervolting attacks like Plundervolt/V0ltPwn. Second, weexperimentally show that overvolting outside the specified range has thepotential of permanently damaging Intel Xeon CPUs, rendering the serverinoperable. We assess the impact of our findings on other server motherboardsmade by Supermicro and ASRock.  Our attacks, dubbed PMFault, can be carried out by a privileged softwareadversary and do not require physical access to the server motherboard orknowledge of the BMC login credentials.  We responsibly disclosed the issues reported in this paper to Supermicro anddiscuss possible countermeasures at different levels. To the best of ourknowledge, the 12th generation of Supermicro motherboards, which was designedbefore we reported PMFault to Supermicro, is not vulnerable.

PMFault: Faulting and Bricking Server CPUs through Management Interfaces