# CVE-2026-64514: userfaultfd: gate must_wait writability check on pte_present()

> A local attacker can cause a denial of service by triggering a race condition in userfaultfd that leaves threads asleep indefinitely due to incorrect PTE checks.

- **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:** 25 Jul 2026
- **Updated:** 17 Aug 2026
- **Also known as:** AZL-93608, BELL-CVE-2026-64514, DEBIAN-CVE-2026-64514, GHSA-r2gx-xm5r-33f5, ROOT-OS-UBUNTU-2204-CVE-2026-64514, UBUNTU-CVE-2026-64514

## What is CVE-2026-64514?

CVE-2026-64514 is a denial of service vulnerability in Linux, published on 25 Jul 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: gate must_wait writability check on pte_present()

userfaultfd_must_wait() and userfaultfd_huge_must_wait() read the PTE
without taking the page table lock and then apply pte_write() /
huge_pte_write() to it.  Those accessors decode bits from the present
encoding only; on a swap or migration entry they read the offset bits that
happen to share the same position and return an undefined result.

The intent of the check is "is this fault still WP-blocked?".  A
non-marker swap entry means the page is in transit -- the userfault
context the original fault delivered against is no longer the same, and
the swap-in or migration completion path will re-deliver a fresh fault if
userspace still needs to handle it.  Worst case under the current code the
garbage write bit says "wait", and the thread stays asleep until a
UFFDIO_WAKE that may never arrive.

Gate the writability check on pte_present() so the lockless re-check only
inspects present-PTE bits when the entry is actually present.  The
non-present, non-marker case returns "don't wait" and lets the fault path
retry.

## Which versions are affected by CVE-2026-64514?


## How do I fix CVE-2026-64514?

Apply the vendor's patch.

## Is CVE-2026-64514 being exploited?

There is no known exploitation of CVE-2026-64514 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64514 and fix it if it is affected.

What it is: CVE-2026-64514 is a denial of service vulnerability in Linux, published on 25 Jul 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-2026-64514 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-2026-64514/ (Markdown: https://insights.sbomify.com/CVE-2026-64514/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64514
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64514
- OSV: https://osv.dev/vulnerability/CVE-2026-64514

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64514/
Summaries are generated and may contain mistakes; the official description above is the record.