Security Version Number Mutable to Older Versions

Security-version number in hardware is mutable, resulting in the ability to downgrade (roll-back) the boot firmware to vulnerable code versions.


Description

A System-on-Chip (SoC) implements secure boot or verified boot. It might support a security version number, which prevents downgrading the current firmware to a vulnerable version. Once downgraded to a previous version, an adversary can launch exploits on the SoC and thus compromise the security of the SoC. These downgrade attacks are also referred to as roll-back attacks.

The security version number must be stored securely and persistently across power-on resets. A common weakness is that the security version number is modifiable by an adversary, allowing roll-back or downgrade attacks or, under certain circumstances, preventing upgrades (i.e. Denial-of-Service on upgrades). In both cases, the SoC is in a vulnerable state.

Demonstrations

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.

Example One

A new version of firmware is signed with a security version number higher than the previous version. During the firmware update process the SoC checks for the security version number and upgrades the SoC firmware with the latest version. This security version number is stored in persistent memory upon successful upgrade for use across power-on resets.

In general, if the security version number is mutable, the implmentation is vulnerable. A mutable security version number allows an adversary to change the security version to a lower value to allow roll-back or to a higher value to prevent future upgrades.

The security version number should be stored in immutable hardware such as fuses, and the writes to these fuses should be highly access-controlled with appropriate authentication and authorization protections.

See Also

Security Flow Issues

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 ...

Comprehensive CWE Dictionary

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Weaknesses without Software Fault Patterns

CWE identifiers in this view are weaknesses that do not have associated Software Fault Patterns (SFPs), as covered by the CWE-888 view. As such, they represent gaps in...

Weaknesses Introduced During Implementation

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