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

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

Which versions are affected by CVE-2023-4921?

It affects 2 products.

Products include Debian Debian Linux, Linux kernel.

How do I fix CVE-2023-4921?

Apply the vendor's patch.

Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.

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 ProFTPD Improper Access Control Exploited Critical 10.0
• CVE-2025-49113 RoundCube Webmail Deserialization of Untrusted Data Exploited Critical 9.9
• CVE-2026-24061 GNU InetUtils Argument Injection Exploited Critical 9.8
• CVE-2025-39682 Linux Kernel Improper Check for Unusual or Exceptional Conditions Exploited Critical 9.8
• CVE-2024-9680 Mozilla Firefox Use-After-Free Exploited Critical 9.8

All Debian Debian Linux vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as AZL-28779, BELL-CVE-2023-4921, DEBIAN-CVE-2023-4921, GHSA-qw4m-2pv2-7r5f, UBUNTU-CVE-2023-4921.