# CVE-2025-40210: Revert "NFSD: Remove the cap on number of operations per NFSv4 COMPOUND"

> An attacker can cause a denial of service in the Linux kernel NFSD by sending an NFSv4 COMPOUND request with an excessively large operation count, triggering a memory allocation failure.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/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:** 21 Nov 2025
- **Updated:** 5 Aug 2026
- **Also known as:** AZL-70660, BELL-CVE-2025-40210, DEBIAN-CVE-2025-40210, GHSA-7q3v-wc68-53r6, ROOT-OS-DEBIAN-11-CVE-2025-40210, ROOT-OS-DEBIAN-12-CVE-2025-40210, ROOT-OS-DEBIAN-13-CVE-2025-40210, ROOT-OS-UBUNTU-2204-CVE-2025-40210, ROOT-OS-UBUNTU-2404-CVE-2025-40210, UBUNTU-CVE-2025-40210

## What is CVE-2025-40210?

CVE-2025-40210 is a high-severity denial of service vulnerability in Linux, published on 21 Nov 2025. 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:

Revert "NFSD: Remove the cap on number of operations per NFSv4 COMPOUND"

I've found that pynfs COMP6 now leaves the connection or lease in a
strange state, which causes CLOSE9 to hang indefinitely. I've dug
into it a little, but I haven't been able to root-cause it yet.
However, I bisected to commit 48aab1606fa8 ("NFSD: Remove the cap on
number of operations per NFSv4 COMPOUND").

Tianshuo Han also reports a potential vulnerability when decoding
an NFSv4 COMPOUND. An attacker can place an arbitrarily large op
count in the COMPOUND header, which results in:

[   51.410584] nfsd: vmalloc error: size 1209533382144, exceeds total
pages, mode:0xdc0(GFP_KERNEL|__GFP_ZERO),
nodemask=(null),cpuset=/,mems_allowed=0

when NFSD attempts to allocate the COMPOUND op array.

Let's restore the operation-per-COMPOUND limit, but increased to 200
for now.

## Which versions are affected by CVE-2025-40210?


## How do I fix CVE-2025-40210?

Apply the vendor's patch.

## Is CVE-2025-40210 being exploited?

There is no known exploitation of CVE-2025-40210 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40210 and fix it if it is affected.

What it is: CVE-2025-40210 is a high-severity denial of service vulnerability in Linux, published on 21 Nov 2025. 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-2025-40210 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-2025-40210/ (Markdown: https://insights.sbomify.com/CVE-2025-40210/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40210
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40210
- OSV: https://osv.dev/vulnerability/CVE-2025-40210

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40210/
Summaries are generated and may contain mistakes; the official description above is the record.