# CVE-2025-39766: net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit

> A logic error in the Linux kernel's CAKE qdisc can trigger a kernel warning and potential denial of service when specific traffic shaping configurations are used.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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:** 11 Sep 2025
- **Updated:** 11 Jun 2026
- **Also known as:** AZL-67232, AZL-73962, BELL-CVE-2025-39766, DEBIAN-CVE-2025-39766, GHSA-p46x-ph7h-c256, ROOT-OS-DEBIAN-11-CVE-2025-39766, ROOT-OS-DEBIAN-12-CVE-2025-39766, ROOT-OS-DEBIAN-13-CVE-2025-39766, ROOT-OS-UBUNTU-2204-CVE-2025-39766, ROOT-OS-UBUNTU-2404-CVE-2025-39766, UBUNTU-CVE-2025-39766

## What is CVE-2025-39766?

CVE-2025-39766 is a high-severity denial of service vulnerability in Debian Debian Linux, published on 11 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:

net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit

The following setup can trigger a WARNING in htb_activate due to
the condition: !cl->leaf.q->q.qlen

tc qdisc del dev lo root
tc qdisc add dev lo root handle 1: htb default 1
tc class add dev lo parent 1: classid 1:1 \
       htb rate 64bit
tc qdisc add dev lo parent 1:1 handle f: \
       cake memlimit 1b
ping -I lo -f -c1 -s64 -W0.001 127.0.0.1

This is because the low memlimit leads to a low buffer_limit, which
causes packet dropping. However, cake_enqueue still returns
NET_XMIT_SUCCESS, causing htb_enqueue to call htb_activate with an
empty child qdisc. We should return NET_XMIT_CN when packets are
dropped from the same tin and flow.

I do not believe return value of NET_XMIT_CN is necessary for packet
drops in the case of ack filtering, as that is meant to optimize
performance, not to signal congestion.

## Which versions are affected by CVE-2025-39766?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-39766?

Apply the vendor's patch.

## Is CVE-2025-39766 being exploited?

There is no known exploitation of CVE-2025-39766 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-2025-39766 and fix it if it is affected.

What it is: CVE-2025-39766 is a high-severity denial of service vulnerability in Debian Debian Linux, published on 11 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-2025-39766 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-2025-39766/ (Markdown: https://insights.sbomify.com/CVE-2025-39766/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39766
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39766
- OSV: https://osv.dev/vulnerability/CVE-2025-39766

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39766/
Summaries are generated and may contain mistakes; the official description above is the record.