Mutable Attestation or Measurement Reporting Data
The register contents used for attestation or measurement reporting data to verify boot flow are modifiable by an adversary.
A System-on-Chip (SoC) implements secure boot or verified boot. During this boot flow, the SoC often measures the code that it authenticates. The measurement is usually done by calculating the one-way hash of the code binary and extending it to the previous hash. The hashing algorithm should be a Secure One-Way hash function. The final hash, i.e., the value obtained after the completion of the boot flow, serves as the measurement data used in reporting or in attestation. The calculated hash is often stored in registers that can later be read by the party of interest to determine tampering of the boot flow. A common weakness is that the contents in these registers are modifiable by an adversary, thus spoofing the measurement.
The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.
Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.
The SoC extends the hash and stores the results in registers. Without protection, an adversary can write their chosen hash values to these registers. Thus, the attacker controls the reported results.
To prevent the above scenario, the registers should have one or more of the following properties:
Should be Read-Only with respect to an adversary
Cannot be extended or modifiable either directly or indirectly (using a trusted agent as proxy) by an adversary
Should have appropriate access controls or protections
Weaknesses in this category are related to improper design of full-system security flows, including but not limited to secure boot, secure update, and hardware-device ...
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