# CVE-2025-21696: mm: clear uffd-wp PTE/PMD state on mremap()

> A kernel inconsistency in userfaultfd write-protect handling during mremap can trigger a page table check warning, potentially causing a denial of service.

- **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:** 12 Feb 2025
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-56839, BELL-CVE-2025-21696, DEBIAN-CVE-2025-21696, GHSA-6q8c-8rxm-f6fh, ROOT-OS-DEBIAN-11-CVE-2025-21696, ROOT-OS-DEBIAN-12-CVE-2025-21696, ROOT-OS-UBUNTU-2204-CVE-2025-21696, ROOT-OS-UBUNTU-2404-CVE-2025-21696, UBUNTU-CVE-2025-21696

## What is CVE-2025-21696?

CVE-2025-21696 is a medium-severity vulnerability in Linux kernel, published on 12 Feb 2025. 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:

mm: clear uffd-wp PTE/PMD state on mremap()

When mremap()ing a memory region previously registered with userfaultfd as
write-protected but without UFFD_FEATURE_EVENT_REMAP, an inconsistency in
flag clearing leads to a mismatch between the vma flags (which have
uffd-wp cleared) and the pte/pmd flags (which do not have uffd-wp
cleared).  This mismatch causes a subsequent mprotect(PROT_WRITE) to
trigger a warning in page_table_check_pte_flags() due to setting the pte
to writable while uffd-wp is still set.

Fix this by always explicitly clearing the uffd-wp pte/pmd flags on any
such mremap() so that the values are consistent with the existing clearing
of VM_UFFD_WP.  Be careful to clear the logical flag regardless of its
physical form; a PTE bit, a swap PTE bit, or a PTE marker.  Cover PTE,
huge PMD and hugetlb paths.

## Which versions are affected by CVE-2025-21696?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21696?

Apply the vendor's patch.

## Is CVE-2025-21696 being exploited?

There is no known exploitation of CVE-2025-21696 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-2025-21696 and fix it if it is affected.

What it is: CVE-2025-21696 is a medium-severity vulnerability in Linux kernel, published on 12 Feb 2025. 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-2025-21696 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-2025-21696/ (Markdown: https://insights.sbomify.com/CVE-2025-21696/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21696
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21696
- OSV: https://osv.dev/vulnerability/CVE-2025-21696

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21696/
Summaries are generated and may contain mistakes; the official description above is the record.