# CVE-2025-38721: netfilter: ctnetlink: fix refcount leak on table dump

> A reference count leak in the Linux kernel netfilter module can cause a resource leak, leading to a denial of service by blocking network namespace cleanup.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 4 Sep 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-66869, AZL-73896, BELL-CVE-2025-38721, DEBIAN-CVE-2025-38721, GHSA-6237-rh4h-82p6, ROOT-OS-DEBIAN-11-CVE-2025-38721, ROOT-OS-DEBIAN-12-CVE-2025-38721, ROOT-OS-UBUNTU-2204-CVE-2025-38721, ROOT-OS-UBUNTU-2404-CVE-2025-38721, UBUNTU-CVE-2025-38721

## What is CVE-2025-38721?

CVE-2025-38721 is a medium-severity denial of service vulnerability in Debian Debian Linux, published on 4 Sep 2025. 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:

netfilter: ctnetlink: fix refcount leak on table dump

There is a reference count leak in ctnetlink_dump_table():
      if (res < 0) {
                nf_conntrack_get(&ct->ct_general); // HERE
                cb->args[1] = (unsigned long)ct;
                ...

While its very unlikely, its possible that ct == last.
If this happens, then the refcount of ct was already incremented.
This 2nd increment is never undone.

This prevents the conntrack object from being released, which in turn
keeps prevents cnet->count from dropping back to 0.

This will then block the netns dismantle (or conntrack rmmod) as
nf_conntrack_cleanup_net_list() will wait forever.

This can be reproduced by running conntrack_resize.sh selftest in a loop.
It takes ~20 minutes for me on a preemptible kernel on average before
I see a runaway kworker spinning in nf_conntrack_cleanup_net_list.

One fix would to change this to:
        if (res < 0) {
		if (ct != last)
	                nf_conntrack_get(&ct->ct_general);

But this reference counting isn't needed in the first place.
We can just store a cookie value instead.

A followup patch will do the same for ctnetlink_exp_dump_table,
it looks to me as if this has the same problem and like
ctnetlink_dump_table, we only need a 'skip hint', not the actual
object so we can apply the same cookie strategy there as well.

## Which versions are affected by CVE-2025-38721?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-38721?

Apply the vendor's patch.

## Is CVE-2025-38721 being exploited?

There is no known exploitation of CVE-2025-38721 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-38721 and fix it if it is affected.

What it is: CVE-2025-38721 is a medium-severity denial of service vulnerability in Debian Debian Linux, published on 4 Sep 2025. 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-2025-38721 in the wild and no public exploit code.

Affected product: Debian Debian 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-2025-38721/ (Markdown: https://insights.sbomify.com/CVE-2025-38721/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-38721
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-38721
- OSV: https://osv.dev/vulnerability/CVE-2025-38721

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-38721/
Summaries are generated and may contain mistakes; the official description above is the record.