BIOS Authenticated Code Module (ACM) for a broad set of Intel processors, including Xeon Scalable: Improper access

Impact

Improper access control (plus, in the sibling CVEs, an out-of-bounds write and an input-validation flaw) in the BIOS ACM. ACMs are Intel-signed code that runs in the authenticated-code execution mode used to establish the root of trust for Boot Guard and TXT - it is more privileged than SMM and more privileged than the hypervisor. Code execution or state corruption at ACM level lets an attacker subvert the measurement chain from underneath, which means a platform can present a valid measured-boot report while running attacker-controlled firmware. Persistence at this level is below-the-OS, survives reimage, and defeats attestation-based tenant-handoff checks.

Who can reach it

A privileged local user - local root or SMM-capable code on the host. On bare-metal GPU nodes handed to tenants with root, that is the tenant.

What to do

The ACM ships inside the BIOS image, so the fix is a BIOS/platform-firmware update from the OEM (Dell, HPE, Supermicro, Lenovo, Gigabyte, Quanta, Wiwynn) - reboot and job drain, and for a May 2022 Intel advisory the OEM server BIOS releases spread across the rest of 2022. There is no configuration workaround: you cannot disable the ACM. If your fleet's trust story depends on Boot Guard or TXT measurements, treat un-updated nodes as not attestable.

References