# CVE-2026-89659: NFSD: Prevent client use-after-free during delegation revoke

> A use-after-free in the Linux kernel's NFSD allows attackers to trigger memory corruption during delegation revocation, potentially leading to code execution or privilege escalation.

- **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:** 21 Oct 2026
- **Also known as:** AZL-100350, BELL-CVE-2026-89659, DEBIAN-CVE-2026-89659, GHSA-vgf3-w23r-qhq2, UBUNTU-CVE-2026-89659

## What is CVE-2026-89659?

CVE-2026-89659 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: Prevent client use-after-free during delegation revoke

A delegation stateid holds only a bare pointer to its owning
nfs4_client and does not keep it alive.  The client survives its
stateids only because __destroy_client() drains cl_delegations and
cl_revoked before free_client() runs.

nfs4_laundromat() breaks that invariant: it unhashes an
expired delegation from cl_delegations, drops deleg_lock, then
revoke_delegation() relinks it onto cl_revoked under cl_lock.  In that
window the delegation is on neither list, so client_has_state() can
report no remaining state.

Every teardown path first requires cl_rpc_users to be zero, but
the laundromat holds no such reference.  A client whose recalled
delegation has just timed out can therefore reach free_client()
while revoke_delegation() is still about to dereference cl_lock,
a use-after-free.

Pin the client with cl_rpc_users across the revoke so teardown blocks
until it completes, then reap the delegation from cl_revoked.  A client
already expiring reaps its own, so skip it and leave the delegation on
del_recall_lru.

## Which versions are affected by CVE-2026-89659?


## How do I fix CVE-2026-89659?

Apply the vendor's patch.

## Is CVE-2026-89659 being exploited?

There is no known exploitation of CVE-2026-89659 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89659 and fix it if it is affected.

What it is: CVE-2026-89659 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-2026-89659 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-2026-89659/ (Markdown: https://insights.sbomify.com/CVE-2026-89659/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89659
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89659
- OSV: https://osv.dev/vulnerability/CVE-2026-89659

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89659/
Summaries are generated and may contain mistakes; the official description above is the record.