CWE-1224

Base Abstraction Level
Pillar — Highest-level weakness category
Class — Abstract, language-independent
Base — Specific enough to detect
Variant — Tied to specific technology
Compound — Requires multiple weaknesses Incomplete MITRE CWE Status
Stable — Fully reviewed and complete
Draft — Under development, may change
Incomplete — Partially defined by MITRE
Deprecated — No longer recommended
Obsolete — Replaced by another CWE
Improper Restriction of Write-Once Bit Fields

Description

The hardware design control register "sticky bits" or write-once bit fields are improperly implemented, such that they can be reprogrammed by software.

Integrated circuits and hardware IP software programmable controls and settings are commonly stored in register circuits. These register contents have to be initialized at hardware reset to define default values that are hard coded in the hardware description language (HDL) code of the hardware unit. A common security protection method used to protect register settings from modification by software is to make the settings write-once or "sticky." This allows writing to such registers only once, whereupon they become read-only. This is useful to allow initial boot software to configure systems settings to secure values while blocking runtime software from modifying such hardware settings. Failure to implement write-once restrictions in hardware design can expose such registers to being re-programmed by software and written multiple times. For example, write-once fields could be implemented to only be write-protected if they have been set to value "1", wherein they would work as "write-1-once" and not "write-once".

Consequences

Confidentiality, Integrity, Availability, Access Control — Varies by Context

System configuration cannot be programmed in a secure way.

Mitigations

Phase: Architecture and Design

During hardware design, all register write-once or sticky fields must be evaluated for proper configuration.

Detection

Automated Analysis

Use automated tools to test that values are not reprogrammable and that write-once fields lock on writing zeros.