CVE-2026-63941
HIGHKVM: arm64: Correctly cap ZCR_EL2 provided by a guest hypervisor
Blast Radius
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Description
In the Linux kernel, the following vulnerability has been resolved:
KVM: arm64: Correctly cap ZCR_EL2 provided by a guest hypervisor
ZCR_EL2 can be updated by a VHE guest hypervisor either using ZCR_EL2 (which traps) or ZCR_EL1 (which does not trap). KVM handles both in different way:
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on ZCR_EL2 trap, ZCR_EL2.LEN is immediately capped at the VM's own VL limit. This has the potential to break existing SW that relies on the full LEN field to be stateful.
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on ZCR_EL1 access, we do absolutely nothing.
On restoring the SVE context for an L2 guest, we directly restore the guest hypervisor's view of ZCR_EL2 into the physical ZCR_EL2. If the guest's view of the register was updated using the ZCR_EL2 accessor, the value has already been sanitised (with the caveat mentioned above).
But if the guest used ZCR_EL1, the raw value is written into the HW, and the L2 guest can now access VLs that it shouldn't.
Fix all the above by moving the VL capping to the restore points, ensuring that:
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the HW is always programmed with a capped value, irrespective of the accessor being used,
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the ZCR_EL2.LEN field is always completely stateful, irrespective of the accessor being used.
Additionally, move ZCR_EL2 to be a sanitised register, ensuring that only the LEN field is actually stateful. This requires some creative construction of the RES0 mask, as the sysreg generation script does not yet generate RAZ/WI fields.
[maz: rewrote commit message, tidy up access_zcr_el2()]
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐧Linux | Kernel | ≥ 6.11.0&&< 7.0.12 | 7.0.12 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Kernel. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update Kernel to 7.0.12 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-63941 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-63941 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-63941. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-63941 in your dependencies?
O3 detects CVE-2026-63941 across Linux dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.