Security Version Number Mutable to Older Versions

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.