Weakness · Medium
CWE-1224: Improper Restriction of Write-Once Bit Fields
What it is
The hardware design control register "sticky bits" or write-once bit fields are improperly implemented, such that they can be reprogrammed by software.
- Severity (our rating)
- Medium
- Likelihood of exploit
- Not rated by MITRE
- Introduced during
- Architecture and Design, Implementation
- Exploited in the wild
- No CVE in CISA's catalog
In more detail
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".
What an attacker can do
- Varies by Context (Confidentiality, Integrity, Availability, Access Control)
How to prevent it
Architecture and Design
- During hardware design, all register write-once or sticky fields must be evaluated for proper configuration.
How to find it
- Automated Analysis. Use automated tools to test that values are not reprogrammable and that write-once fields lock on writing zeros.
H2 Security
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Start freeSource: MITRE Common Weakness Enumeration, CWE version 4.19.1. CWE is a trademark of The MITRE Corporation.