# CVE-2026-89712: NFSD: restart ssc_expire_umount walk after dropping nfsd_ssc_lock

> A race condition in the Linux kernel NFS server allows a use-after-free, potentially leading to code execution or denial of service.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100715, BELL-CVE-2026-89712, DEBIAN-CVE-2026-89712, GHSA-cfxc-m2r2-5grw, UBUNTU-CVE-2026-89712

## What is CVE-2026-89712?

CVE-2026-89712 is a critical use after free vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

NFSD: restart ssc_expire_umount walk after dropping nfsd_ssc_lock

nfsd4_ssc_expire_umount() walks nn->nfsd_ssc_mount_list with
list_for_each_entry_safe(ni, tmp, ...).  For each expired entry it
sets nsui_busy = true, drops nfsd_ssc_lock to run mntput() on the
source vfsmount, then reacquires the lock to list_del + kfree the
entry and continue iterating via the macro's saved tmp pointer.

The nsui_busy flag protects the current ni from concurrent
nfsd4_ssc_setup_dul() finders during the lock-drop window, but it
does not pin tmp.  Another nfsd RPC thread that fails its source-
server mount and reaches nfsd4_ssc_cancel_dul() will, during that
same window, take nfsd_ssc_lock, list_del + kfree its own ssc_umount
item, and release the lock.  If that item is the saved tmp of the
expire walk, the next iteration dereferences a freed
nfsd4_ssc_umount_item.

Restart the walk from the head after the mntput() unlock window so
no saved next pointer survives the lock-drop.  The list is bounded
by the number of active inter-server source mounts (typically small)
and the expire delayed-work runs periodically rather than per-IO,
so the restart is cheap.

## Which versions are affected by CVE-2026-89712?


## How do I fix CVE-2026-89712?

Apply the vendor's patch.

## Is CVE-2026-89712 being exploited?

There is no known exploitation of CVE-2026-89712 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89712 and fix it if it is affected.

What it is: CVE-2026-89712 is a critical use after free vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89712 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-2026-89712/ (Markdown: https://insights.sbomify.com/CVE-2026-89712/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89712
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89712
- OSV: https://osv.dev/vulnerability/CVE-2026-89712

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89712/
Summaries are generated and may contain mistakes; the official description above is the record.