Security

Peripheral firmware security

Mac computers have many built-in peripheral processors dedicated to tasks such as networking, graphics, power management, or managing data buses like USB or Thunderbolt. Often peripheral firmware is single-purpose, and much less powerful than the Intel CPU. However, built-in peripherals that don’t implement sufficient security become a target for attackers seeking even easier targets to exploit and then persistently infect the operating system. Having infected a peripheral processor firmware, an attacker could target software on the Intel CPU, or directly capture sensitive data (for example, an Ethernet device could see the contents of packets which aren’t encrypted).

Apple strategically works with third-party vendors to reduce (wherever possible) the number of peripheral processors necessary, or to avoid designs that require firmware. But when firmware is required, efforts are taken to ensure an attacker can’t persist on that processor. This can be achieved:

By running the processor in a mode where it downloads verified firmware from the Intel CPU on startup
By ensuring the peripheral processor implements its own secure boot chain where it verifies its own firmware on every boot

Apple works with vendors to audit their implementations, and enhance their designs to include desired properties such as:

Ensuring minimum cryptographic strengths
Strong revocation of known bad firmware
Disabling debug interfaces
Signing the firmware with cryptographic keys that are stored in Apple-controlled Hardware Security Modules (HSMs)

In recent years Apple has worked with some external vendors to adopt the same “Image4” data structures, verification code, and signing infrastructure used by iOS, iPadOS, and Mac computers with the Apple T2 Security Chip.

When neither storage-free operation nor storage plus secure boot is an option, the design mandates that firmware updates be cryptographically signed and verified before the persistent storage can be updated.

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Apple Platform Security

Peripheral firmware security