# CVE-2026-31421: net/sched: cls_fw: fix NULL pointer dereference on shared blocks

> A NULL pointer dereference in the Linux kernel's traffic control classifier allows local attackers to cause a system crash by attaching specific filters to shared blocks.

- **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:** 13 Apr 2026
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-82721, BELL-CVE-2026-31421, DEBIAN-CVE-2026-31421, GHSA-pmqm-g7jm-76hf, ROOT-OS-DEBIAN-11-CVE-2026-31421, ROOT-OS-DEBIAN-12-CVE-2026-31421, ROOT-OS-DEBIAN-13-CVE-2026-31421, ROOT-OS-UBUNTU-2204-CVE-2026-31421, ROOT-OS-UBUNTU-2404-CVE-2026-31421, UBUNTU-CVE-2026-31421

## What is CVE-2026-31421?

CVE-2026-31421 is a medium-severity vulnerability in Linux kernel, published on 13 Apr 2026. 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: cls_fw: fix NULL pointer dereference on shared blocks

The old-method path in fw_classify() calls tcf_block_q() and
dereferences q->handle.  Shared blocks leave block->q NULL, causing a
NULL deref when an empty cls_fw filter is attached to a shared block
and a packet with a nonzero major skb mark is classified.

Reject the configuration in fw_change() when the old method (no
TCA_OPTIONS) is used on a shared block, since fw_classify()'s
old-method path needs block->q which is NULL for shared blocks.

The fixed null-ptr-deref calling stack:
 KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]
 RIP: 0010:fw_classify (net/sched/cls_fw.c:81)
 Call Trace:
  tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860)
  tc_run (net/core/dev.c:4401)
  __dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2026-31421?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31421?

Apply the vendor's patch.

## Is CVE-2026-31421 being exploited?

There is no known exploitation of CVE-2026-31421 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31421 and fix it if it is affected.

What it is: CVE-2026-31421 is a medium-severity vulnerability in Linux kernel, published on 13 Apr 2026. 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-2026-31421 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-31421/ (Markdown: https://insights.sbomify.com/CVE-2026-31421/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31421
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31421
- OSV: https://osv.dev/vulnerability/CVE-2026-31421

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31421/
Summaries are generated and may contain mistakes; the official description above is the record.