# CVE-2026-89661: NFSD: Prevent post-shutdown use-after-free in unlock_filesystem

> A local administrator can trigger a use-after-free in the Linux kernel NFSD module by writing to a proc file after server shutdown, potentially causing a crash or code execution.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-89661, GHSA-5jhw-w88m-cjf3, UBUNTU-CVE-2026-89661

## What is CVE-2026-89661?

CVE-2026-89661 is an use after free vulnerability in Linux, published on 11 Sep 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:

NFSD: Prevent post-shutdown use-after-free in unlock_filesystem

Writing a filesystem path to /proc/fs/nfsd/unlock_filesystem runs
nfsd4_cancel_copy_by_sb() before nfsd_mutex is held and before the
handler confirms that nn->nfsd_serv is set. Once nfsd has shut down,
nfs4_state_destroy_net() has freed nn->conf_id_hashtbl but left the
pointer intact, so the cancel helper iterates freed slab memory as an
array of struct list_head and then dereferences a bogus nfs4_client
when it takes clp->async_lock. A local administrator holding
CAP_SYS_ADMIN can reach this use-after-free by stopping the server and
then writing to unlock_filesystem; KASAN reports a slab-use-after-free
read in nfsd4_cancel_copy_by_sb().

nfsd4_revoke_states() walks the same state tables and for that reason
already runs only under nfsd_mutex with nn->nfsd_serv confirmed
present. Move the async COPY cancel into that protected section so
every NFSv4 state-table walker on this path observes a running server.
Async copies exist only while the server runs, so gating the cancel on
nn->nfsd_serv loses nothing.

## Which versions are affected by CVE-2026-89661?


## How do I fix CVE-2026-89661?

Apply the vendor's patch.

## Is CVE-2026-89661 being exploited?

There is no known exploitation of CVE-2026-89661 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89661 and fix it if it is affected.

What it is: CVE-2026-89661 is an use after free vulnerability in Linux, published on 11 Sep 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-2026-89661 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-2026-89661/ (Markdown: https://insights.sbomify.com/CVE-2026-89661/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89661
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89661
- OSV: https://osv.dev/vulnerability/CVE-2026-89661

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89661/
Summaries are generated and may contain mistakes; the official description above is the record.