# CVE-2026-89711: NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check

> A flawed warning check in the Linux kernel NFSD module can lead to a null pointer dereference during NFS reexports, causing a denial of service.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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-100889, BELL-CVE-2026-89711, DEBIAN-CVE-2026-89711, GHSA-f2q5-7q65-xqx7, UBUNTU-CVE-2026-89711

## What is CVE-2026-89711?

CVE-2026-89711 is a high-severity 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: remove flawed WARN_ON_ONCE from nfsd_mode_check

The header for commit e75b23f9e323 ("nfsd: check d_can_lookup in
fh_verify of directories") details the assumption that justified
adding the WARN_ON_ONCE to nfsd_mode_check(), that assumption is
invalid (in the case of NFS reexport).

When NFSD exports an NFS filesystem it is very possible for
nfsd_mode_check() to encounter a @dentry that doesn't have
i_op->lookup (see nfs_fhget()'s NFS_ATTR_FATTR_MOUNTPOINT and
NFS_ATTR_FATTR_V4_REFERRAL handling, and d_flags_for_inode()).

So remove nfsd_mode_check()'s WARN_ON_ONCE(). The nfserr_notdir
return on that branch must stay. It guards the subsequent
lookup_one_unlocked() -> __lookup_slow() path, which calls
inode->i_op->lookup() with no NULL check, so returning nfserr_notdir
is what keeps a client LOOKUP into such a @dentry from dereferencing
a NULL method pointer.

## Which versions are affected by CVE-2026-89711?


## How do I fix CVE-2026-89711?

Apply the vendor's patch.

## Is CVE-2026-89711 being exploited?

There is no known exploitation of CVE-2026-89711 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89711 and fix it if it is affected.

What it is: CVE-2026-89711 is a high-severity 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-2026-89711 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-2026-89711/ (Markdown: https://insights.sbomify.com/CVE-2026-89711/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89711
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89711
- OSV: https://osv.dev/vulnerability/CVE-2026-89711

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89711/
Summaries are generated and may contain mistakes; the official description above is the record.