# CVE-2026-68166: userfaultfd: prevent registration of special VMAs

> A local attacker can exploit userfaultfd on shadow stack VMAs to inject return addresses, leading to privilege escalation or code execution.

- **Severity:** Not scored
- **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:** 10 Aug 2026
- **Updated:** 23 Aug 2026
- **Also known as:** AZL-94734, BELL-CVE-2026-68166, DEBIAN-CVE-2026-68166, GHSA-hv63-47wx-6p3x, UBUNTU-CVE-2026-68166

## What is CVE-2026-68166?

CVE-2026-68166 is a vulnerability in Linux, published on 10 Aug 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:

userfaultfd: prevent registration of special VMAs

Vova Tokarev says:

  userfaultfd allows registration on shadow stack VMAs.  With userfaultfd
  access, you can register on the shadow stack, discard a page ... and
  inject a page with chosen return addresses via UFFDIO_COPY.

Update vma_can_userfault() to reject VM_SHADOW_STACK.

While on it, also reject VM_SPECIAL so that if a driver would implement
vm_uffd_ops, it wouldn't be possible to register special VMAs with
userfaultfd.

Since VM_SPECIAL includes VM_DONTEXPAND which is set but hugetlb, exclude
hugetlb VMAs from the check for VM_SPECIAL.

## Which versions are affected by CVE-2026-68166?


## How do I fix CVE-2026-68166?

Apply the vendor's patch.

## Is CVE-2026-68166 being exploited?

There is no known exploitation of CVE-2026-68166 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-68166 and fix it if it is affected.

What it is: CVE-2026-68166 is a vulnerability in Linux, published on 10 Aug 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-2026-68166 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-2026-68166/ (Markdown: https://insights.sbomify.com/CVE-2026-68166/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-68166
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-68166
- OSV: https://osv.dev/vulnerability/CVE-2026-68166

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-68166/
Summaries are generated and may contain mistakes; the official description above is the record.