# CVE-2026-99186: x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx()


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 28 Oct 2026
- **Updated:** 28 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-99186, GHSA-252w-9m2h-vq2g, UBUNTU-CVE-2026-99186

## What is CVE-2026-99186?

CVE-2026-99186 is a vulnerability in Linux, published on 28 Oct 2026. 

### Official description

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

x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx()

Commit 45e1dccc0653 ("x86/insn: Simplify for_each_insn_prefix()") made
for_each_insn_prefix() declare its own loop counter named idx.
get_seg_reg_override_idx() keeps its result in a local of the same name,
so the assignments in the loop body now write the loop counter:

 - the function always returns INAT_SEG_REG_DEFAULT, so FS and GS
   overrides are ignored;

 - a CS override sets the counter to INAT_SEG_REG_CS (2), the increment
   makes it 3, and prefixes.bytes[3] holds the last prefix, 0x2e again,
   so the loop never terminates.

The UMIP emulation reaches this from the #GP handler for user space: an
SGDT, SIDT or SMSW with a memory operand and a CS prefix leaves the task
spinning in the kernel, never returning to user space, with a SIGKILL
that is never delivered.  Any user can thus tie up CPUs until reboot.
With an FS or GS prefix, the emulated result is written to the wrong
address.

Rename the internal macro iterator variable.

[ Lukas' original patch renamed the use site, I changed it to rename the
  iterator in the macro instead, but left Lukas' commit message and
  authorship   - Linus ]

## Which versions are affected by CVE-2026-99186?


## How do I fix CVE-2026-99186?

Upgrade to a fixed version.

## Is CVE-2026-99186 being exploited?

There is no known exploitation of CVE-2026-99186 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99186 and fix it if it is affected.

What it is: CVE-2026-99186 is a vulnerability in Linux, published on 28 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99186 in the wild and no public exploit code.

Affected product: Linux. 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-2026-99186/ (Markdown: https://insights.sbomify.com/CVE-2026-99186/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99186
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99186
- OSV: https://osv.dev/vulnerability/CVE-2026-99186

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99186/
Summaries are generated and may contain mistakes; the official description above is the record.