System management mode (SMM) is one example of a special purpose operating mode provided by x86 architecture CPUs. SMM may be used for handling system-wide functions such as power management, system hardware control, or proprietary original equipment manufacturer (OEM)-designed code. It is typically intended for use by system firmware of an OEM, and is not intended to be third party extensible. SMM is considered as a “hidden” operating mode because the operating system (OS) and OS-executed software applications cannot see it or access it.
The presence of SMM as a privileged operating mode in the CPU, however, may allow the possibility that a rootkit or other malicious software could operate undetected in SMM, and gain complete access to system memory and functions. Because SMM is provided as a hardware function, it cannot be disabled in software by the operating system.