UEFI firmware security in an Intel-based Mac
Since 2006, Mac computers with an Intel-based CPU use an Intel firmware based on the Extensible Firmware Interface (EFI) Development Kit (EDK) version 1 or version 2. EDK2-based code conforms to the Unified Extensible Firmware Interface (UEFI) specification. This section refers to the Intel firmware as the UEFI firmware. The UEFI firmware was the first code to execute on the Intel chip.
For an Intel-based Mac without the Apple T2 Security Chip, the root of trust for the UEFI firmware is the chip where the firmware is stored. UEFI firmware updates are digitally signed by Apple and verified by the firmware before updating the storage. To prevent rollback attacks, updates must always have a version newer than the existing one. However, an attacker with physical access to the Mac could potentially use hardware to attach to the firmware storage chip and update the chip to contain malicious content. Likewise, if vulnerabilities are found in the early boot process of the UEFI firmware (before it write-restricts the storage chip), this could also lead to persistent infection of the UEFI firmware. This is a hardware architectural limitation common in most Intel-based PCs and present in all Intel-based Mac computers without the T2 chip.
To help prevent physical attacks that subvert UEFI firmware, Mac computers were rearchitected to root the trust in the UEFI firmware in the T2 chip. On these Mac computers, the root of trust for the UEFI firmware is specifically the T2 firmware, as described in Boot process for an Intel-based Mac.
Intel Management Engine (ME) subcomponent
One subcomponent stored within the UEFI firmware is the Intel Management Engine (ME) firmware. The ME—a separate processor and subsystem within Intel chips—is used primarily for audio and video copyright protection on a Mac that has only Intel-based graphics. To reduce this subcomponent’s attack surface, an Intel-based Mac runs a custom ME firmware from which most components have been removed. Because the resulting Mac ME firmware is smaller than the default minimal build that Intel makes available, many components that have been the subject of public attacks by security researchers in the past are no longer present.
System Management Mode (SMM)
Intel processors have a special execution mode that’s distinct from normal operation. Called System Management Mode (SMM), it was originally introduced to handle time-sensitive operations such as power management. However, to perform such actions, Mac computers have historically used a discrete microcontroller called the System Management Controller (SMC). No longer a separate microcontroller, the SMC has been integrated into the T2 chip.