IBM Power Systems Firmware FW1060.00 through FW1060.81 is affected by a vulnerability in the service processor mailbox interface. An attacker with authenticated service-level access to the FSP can send a specially crafted mailbox message to read or modify arbitrary regions of Hostboot memory, compromising the host firmware boot stack and the hypervisor subsequently loaded by it. Successful exploitation results in a confidentiality, integrity, and availability impact to the managed system.