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2.3 System BIOS Update Mechanisms

NIST platform security: roots of trust, firmware and secure enclaves SP 800-147B
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Title

System BIOS Update Mechanisms

Control

The guidelines in this publication are intended to secure BIOS update mechanisms so that only authentic, authorized BIOS images are written to BIOS flash memory, a process sometimes referred to as “flashing” the image. While client systems typically only have one path for updating the BIOS, server systems may implement several update mechanisms to allow administrators to update the BIOS from different environments. This publication identifies three general types of authenticated BIOS update mechanisms. A server will typically implement one or more of these update mechanisms. Each mechanism is briefly described below:

 Update Mechanism 1 – Authenticated BIOS update that can occur anytime . This update mechanism allows the BIOS flash memory to be securely updated irrespective of the operating state of the server. This includes being able to update the BIOS flash while the server operates, without requiring a reboot. This mechanism must protect the BIOS flash memory from unauthorized modifications.  Update Mechanism 2 – Authenticated BIOS update on reboot . This BIOS update mechanism allows the flash process to be initiated during server runtime, but the actual update of the BIOS flash memory does not occur until the system reboots. Like the previous mechanism, this mechanism must protect the BIOS flash memory from unauthorized modifications.  Update Mechanism 3 – BIOS update at runtime and verification of BIOS on reboot . With this BIOS update mechanism, the BIOS is verified to be authentic before it is executed on every boot. This mechanism is necessary if the BIOS flash locking mechanism is inadequate to protect the integrity of a running system’s BIOS flash memory. If the BIOS flash memory is determined to be inauthentic during boot, a recovery process is initiated and the inauthentic BIOS is not executed.

In addition, some servers may include a secure local update mechanism that updates the system BIOS without using any of the three authenticated update mechanisms discussed above. A secure local update mechanism ensures the authenticity and integrity of the BIOS update image by requiring that an administrator be physically present at the server to conduct the update. The requirement for physical presence mitigates the risk of remote attacks against the system BIOS.