# CVE-2026-90039: NFSD: Guard admin state-revocation walks with NFSD_NET_UP

> A local administrator can crash the Linux kernel by triggering NFS state revocation before server threads start, causing a null pointer dereference.

- **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:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-90039, DEBIAN-CVE-2026-90039, GHSA-m7r7-c74x-x286, UBUNTU-CVE-2026-90039

## What is CVE-2026-90039?

CVE-2026-90039 is a vulnerability in Linux, published on 16 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: Guard admin state-revocation walks with NFSD_NET_UP

Writing to /proc/fs/nfsd/unlock_filesystem, or sending the
NFSD_CMD_UNLOCK_FILESYSTEM or NFSD_CMD_UNLOCK_EXPORT netlink command,
walks the NFSv4 client hash tables to revoke open state and cancel
async COPY operations.  All three handlers gate that walk on
nn->nfsd_serv, but a listener added via portlist or netlink
listener_set sets nn->nfsd_serv before any nfsd thread starts.
nfsd_startup_net() has not yet allocated nn->conf_id_hashtbl, so the
walkers dereference a NULL table.  A local administrator with
CAP_SYS_ADMIN can crash the kernel this way without ever starting the
server.

nn->nfsd_serv is set when the service is created, which precedes
table allocation.  NFSD_NET_UP instead brackets the window where the
tables are live: set at the end of nfsd_startup_net() and cleared in
nfsd_shutdown_net() after they are freed, both under nfsd_mutex.
Gating the three unlock paths on NFSD_NET_UP fixes the startup-time
NULL dereference while preserving the earlier post-shutdown
use-after-free fix.

## Which versions are affected by CVE-2026-90039?


## How do I fix CVE-2026-90039?

Apply the vendor's patch.

## Is CVE-2026-90039 being exploited?

There is no known exploitation of CVE-2026-90039 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90039 and fix it if it is affected.

What it is: CVE-2026-90039 is a vulnerability in Linux, published on 16 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-2026-90039 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-2026-90039/ (Markdown: https://insights.sbomify.com/CVE-2026-90039/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90039
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90039
- OSV: https://osv.dev/vulnerability/CVE-2026-90039

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90039/
Summaries are generated and may contain mistakes; the official description above is the record.