# CVE-2023-4921: Use-after-free in Linux kernel's net/sched: sch_qfq component

> A use-after-free in the Linux kernel's QFQ scheduler allows local attackers to escalate privileges when specific qdisc 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:** 12 Sep 2023
- **Updated:** 14 Oct 2026
- **Also known as:** AZL-28779, BELL-CVE-2023-4921, DEBIAN-CVE-2023-4921, GHSA-qw4m-2pv2-7r5f, UBUNTU-CVE-2023-4921

## What is CVE-2023-4921?

CVE-2023-4921 is a high-severity use after free vulnerability in Debian Debian Linux, published on 12 Sep 2023. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A use-after-free vulnerability in the Linux kernel's net/sched: sch_qfq component can be exploited to achieve local privilege escalation.

When the plug qdisc is used as a class of the qfq qdisc, sending network packets triggers use-after-free in qfq_dequeue() due to the incorrect .peek handler of sch_plug and lack of error checking in agg_dequeue().

We recommend upgrading past commit 8fc134fee27f2263988ae38920bc03da416b03d8.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2023-4921?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2023-4921?

Apply the vendor's patch.

## Is CVE-2023-4921 being exploited?

There is no known exploitation of CVE-2023-4921 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-2023-4921 and fix it if it is affected.

What it is: CVE-2023-4921 is a high-severity use after free vulnerability in Debian Debian Linux, published on 12 Sep 2023. 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-2023-4921 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-2023-4921/ (Markdown: https://insights.sbomify.com/CVE-2023-4921/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-4921
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-4921
- OSV: https://osv.dev/vulnerability/CVE-2023-4921

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-4921/
Summaries are generated and may contain mistakes; the official description above is the record.