UEFI stands for Unified Extensible Firmware Interface, and UEFI
UEFI has replaced the old BIOS system that used to be present in all the previous versions of Windows. The old BIOS system had no such security feature as offered by the UEFI. So, it was very easy for the malware to attack the BIOS, and once the BIOS was infected, the malware would become immune to antivirus installed in the system. Thus, it was very difficult to deal with such malware as they were resident in the boot sector. They would appear again even after new copy of Windows was installed after formatting the system.
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UEFI secure boot in Windows 8 does not allow any malware to load itself into the pre-OS environment. It works on the principle of digital signatures. UEFI comes bundled with some valid public keys. The boot loader, in order to load itself successfully into the pre-OS environment, must contain a private key that matches one of the keys of UEFI. That means the boot loader must be digitally signed by Microsoft.
However, along with secure boot feature UEFI brings one serious disadvantage. You will not be able run Windows 8 and Linux on the same machine. This is obvious because you can’t expect the boot loader of Linux to be digitally signed by Microsoft.