# CVE-2026-31561: x86/cpu: Remove X86_CR4_FRED from the CR4 pinned bits mask

> Attackers can disable CR4 pinning to bypass security features and escalate privileges or cause system crashes in Linux kernels.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Apr 2026
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2026-31561, DEBIAN-CVE-2026-31561, GHSA-pgvp-p3vq-7q7h, ROOT-OS-DEBIAN-13-CVE-2026-31561, UBUNTU-CVE-2026-31561

## What is CVE-2026-31561?

CVE-2026-31561 is a medium-severity vulnerability in Linux kernel, published on 24 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

x86/cpu: Remove X86_CR4_FRED from the CR4 pinned bits mask

Commit in Fixes added the FRED CR4 bit to the CR4 pinned bits mask so
that whenever something else modifies CR4, that bit remains set. Which
in itself is a perfectly fine idea.

However, there's an issue when during boot FRED is initialized: first on
the BSP and later on the APs. Thus, there's a window in time when
exceptions cannot be handled.

This becomes particularly nasty when running as SEV-{ES,SNP} or TDX
guests which, when they manage to trigger exceptions during that short
window described above, triple fault due to FRED MSRs not being set up
yet.

See Link tag below for a much more detailed explanation of the
situation.

So, as a result, the commit in that Link URL tried to address this
shortcoming by temporarily disabling CR4 pinning when an AP is not
online yet.

However, that is a problem in itself because in this case, an attack on
the kernel needs to only modify the online bit - a single bit in RW
memory - and then disable CR4 pinning and then disable SM*P, leading to
more and worse things to happen to the system.

So, instead, remove the FRED bit from the CR4 pinning mask, thus
obviating the need to temporarily disable CR4 pinning.

If someone manages to disable FRED when poking at CR4, then
idt_invalidate() would make sure the system would crash'n'burn on the
first exception triggered, which is a much better outcome security-wise.

## Which versions are affected by CVE-2026-31561?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31561?

Apply the vendor's patch.

## Is CVE-2026-31561 being exploited?

There is no known exploitation of CVE-2026-31561 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31561 and fix it if it is affected.

What it is: CVE-2026-31561 is a medium-severity vulnerability in Linux kernel, published on 24 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-31561 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-31561/ (Markdown: https://insights.sbomify.com/CVE-2026-31561/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31561
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31561
- OSV: https://osv.dev/vulnerability/CVE-2026-31561

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31561/
Summaries are generated and may contain mistakes; the official description above is the record.