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Linux KernelOperating system · Linux

CVE-2023-4921

HIGH · 7.8 CVSS v3.1 Published 2023-09-12
Fix available
A fix is available. Upgrade to 4.14.326 / 4.19.295 or later.
See remediation →
80/100
Remediation priority · High
Public exploit Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis
How this class of weakness works · CWE-416

Memory is used after it has been freed, so its contents — now potentially attacker-controlled — drive the program's behaviour. With careful heap grooming this becomes code execution. The fix requires disciplined ownership of memory and often a targeted rework of the object lifecycle.

General guidance for the use after free class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 3.8, < 4.14.326>= 4.15, < 4.19.295>= 4.20, < 5.4.257>= 5.5, < 5.10.195>= 5.11, < 5.15.132>= 5.16, < 6.1.54>= 6.2, < 6.5.4
Debian LinuxOperating system
Affected:= 10.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 4.14.326 / 4.19.295 / 5.4.257 or later
Fixed in 4.14.3264.19.2955.4.257
Vendor patch git.kernel.org →
Recommended fix High confidence

Kernel 4.14.326+ / 4.19.295+ / 5.4.257+ / 5.10.195+ (depending on your base kernel branch); Debian 11+ if using Debian

  1. 1. Identify the currently running kernel version using `uname -r`
  2. 2. For enterprise Linux distributions (RHEL, CentOS, Ubuntu LTS), check if the vendor has released a kernel update that includes the fix (commit 8fc134fee27f2263988ae38920bc03da416b03d8)
  3. 3. For Debian 10 users: Since Debian 10 (Buster) is now end-of-life, upgrade to Debian 11 (Bullseye) or Debian 12 (Bookworm) to receive kernel updates with the security fix
  4. 4. Apply the kernel upgrade via system package manager: `apt-get update && apt-get upgrade` (Debian/Ubuntu) or `yum update` (RHEL/CentOS)
  5. 5. Reboot the system to load the new kernel: `reboot`
  6. 6. After reboot, verify the new kernel version is running with `uname -r` and confirm it's at or above the fixed version for your kernel branch
Caveat Kernel upgrades may require system reboot and could introduce compatibility issues with proprietary kernel modules or older drivers; Debian 10 to Debian 11/12 upgrade requires system migration review

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $3,200. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2023-4921 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
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