CWE-1251 applies when a hardware architecture mirrors regions without making sure their contents always stay in sync. Hardware designers and architects meet it most, but the underlying idea, a fast copy that must match its source, is familiar to anyone who has debugged a stale cache.
How it happens
Performance often calls for duplication. A cache keeps a local copy of a value in fast memory. The speed gain has a price: the product must make sure the local copy always reflects the original. If they get out of sync, results are no longer true.
In hardware, memory is not the only thing mirrored. Registers, memory regions and sometimes whole computational units get duplicated too.
MITRE's example uses a processor whose main Memory Management Unit has five shadow MMUs. Whenever the accessible range changes with boot status, the main MMU sends an update message to each shadow. If the interconnect does not prioritize update packets over general traffic, a race condition exists. An attacker who floods the target with enough messages can get attack packets through before the new access ranges are applied.
MITRE states the core problem plainly. When there are several physically different copies of a value, and the update process is not instantaneous and atomic, you cannot claim the copies are always in sync. There will always be a time period when they are not.
What an attacker gains
The record lists consequences across confidentiality, integrity, availability, access control, authentication, authorization, accountability and non-repudiation, varying by context. Mirrored regions with different values can expose sensitive information or lead to system compromise.
How to find it
The record lists no detection methods. A review should:
- Inventory every mirrored or shadowed item: registers, memory regions, caches, whole units.
- For each one, identify the update path and the time between a change and its arrival.
- Ask what each copy controls and what an attacker could do inside the window.
- Test whether update traffic can be delayed by competing traffic.
How to fix it
MITRE's mitigation, rated moderate in effectiveness, is honest about its limits. Since perfect sync cannot be asserted, the recommendations are to make the out-of-sync period as short as possible, and to make the update process as dependable as possible.
Quick check
Can you guarantee mirrored copies are always identical?
Not if the update is not instantaneous and atomic. MITRE says there will always be a period when the copies are out of sync.
What are the two mitigation goals?
Make the out-of-sync window as small as possible, and make the update process as dependable as possible.
