# CVE-2026-90036: NFSD: Prevent client use-after-free during blocked-lock reaping

> A use-after-free in the Linux kernel NFSD module allows attackers to execute arbitrary code or escalate privileges via blocked-lock reaping.

- **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:** 16 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-101540, BELL-CVE-2026-90036, DEBIAN-CVE-2026-90036, GHSA-j6h5-vv56-hq58, UBUNTU-CVE-2026-90036

## What is CVE-2026-90036?

CVE-2026-90036 is a critical use after free vulnerability in Linux, published on 16 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 blocked-lock reaping

A bare lock owner -- its only remaining reference a blocked lock on
nn->blocked_locks_lru -- holds a raw pointer to its nfs4_client but
no reference keeping the client alive. When the per-net laundromat
reaps such a lock, freeing the nbl drops the owner reference
held through flc_owner, and the final nfs4_put_stateowner()
takes the client's cl_lock. Because the laundromat detaches the
nbl first, __destroy_client() no longer finds it, so a concurrent
force_expire_client() can free the client before nfs4_put_stateowner()
runs, dereferencing cl_lock in freed memory.

Pin the client with cl_rpc_users before dropping
nn->blocked_locks_lock, and skip clients already expiring, whose
blocked locks __destroy_client() frees while holding an owner
reference. Take nn->client_lock outside nn->blocked_locks_lock.
Every other site holds nn->blocked_locks_lock as a leaf, acquiring
no further lock, so placing nn->client_lock outside it cannot form
a lock-order cycle.

## Which versions are affected by CVE-2026-90036?


## How do I fix CVE-2026-90036?

Apply the vendor's patch.

## Is CVE-2026-90036 being exploited?

There is no known exploitation of CVE-2026-90036 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90036 and fix it if it is affected.

What it is: CVE-2026-90036 is a critical use after free vulnerability in Linux, published on 16 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-2026-90036 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-2026-90036/ (Markdown: https://insights.sbomify.com/CVE-2026-90036/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90036
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90036
- OSV: https://osv.dev/vulnerability/CVE-2026-90036

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90036/
Summaries are generated and may contain mistakes; the official description above is the record.