---
id: CVE-2025-21703
title: |-
  In the Linux kernel, the following vulnerability has been resolved:

  netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

  qdisc_tree_reduce_backlog() notifies parent qdisc only if child
  qdisc becomes empty, therefore we need to …
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

  qdisc_tree_reduce_backlog() notifies parent qdisc only if child
  qdisc becomes empty, therefore we need to …
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
cwe:
  - CWE-416
vendor: linux
product: linux_kernel
affected:
  - 'linux_kernel >= 5.4.288, < 5.4.291'
  - 'linux_kernel >= 5.10.232, < 5.10.235'
  - 'linux_kernel >= 5.15.175, < 5.15.179'
  - 'linux_kernel >= 6.1.121, < 6.1.129'
  - 'linux_kernel >= 6.6.67, < 6.6.78'
  - 'linux_kernel >= 6.12.6, < 6.12.14'
  - 'linux_kernel >= 6.13, < 6.13.3'
  - linux_kernel = 6.14
patched:
  - linux_kernel 6.13.3
published: '2025-02-18'
updated: '2026-10-15'
sourceUpdated: '2026-10-15T02:17:28.087'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2025-21703'
references:
  - url: 'https://git.kernel.org/stable/c/1f8e3f4a4b8b90ad274dfbc66fc7d55cb582f4d5'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/6312555249082d6d8cc5321ff725df05482d8b83'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/638ba5089324796c2ee49af10427459c2de35f71'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/7b79ca9a1de6a428d486ff52fb3d602321c08f55'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/7f31d74fcc556a9166b1bb20515542de7bb939d1'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/839ecc583fa00fab785fde1c85a326743657fd32'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/98a2c685293aae122f688cde11d9334dddc5d207'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://lists.debian.org/debian-lts-announce/2025/03/msg00028.html'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://cert-portal.siemens.com/productcert/html/ssa-503939.html'
    label: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e
  - url: 'https://cert-portal.siemens.com/productcert/html/ssa-784472.html'
    label: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e
tags:
  - nvd
  - cve.org
epss: 0.00304
epssPercentile: 0.21632
ingestedAt: '2026-10-15T03:30:12.785Z'
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2025-02-18T15:38:37.163490Z'
---

## Overview

In the Linux kernel, the following vulnerability has been resolved:

netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

qdisc_tree_reduce_backlog() notifies parent qdisc only if child
qdisc becomes empty, therefore we need to reduce the backlog of the
child qdisc before calling it. Otherwise it would miss the opportunity
to call cops->qlen_notify(), in the case of DRR, it resulted in UAF
since DRR uses ->qlen_notify() to maintain its active list.

## Affected

- `linux_kernel >= 5.4.288, < 5.4.291`
- `linux_kernel >= 5.10.232, < 5.10.235`
- `linux_kernel >= 5.15.175, < 5.15.179`
- `linux_kernel >= 6.1.121, < 6.1.129`
- `linux_kernel >= 6.6.67, < 6.6.78`
- `linux_kernel >= 6.12.6, < 6.12.14`
- `linux_kernel >= 6.13, < 6.13.3`
- `linux_kernel = 6.14`

## Remediation

Upgrade past the affected range:

- `linux_kernel 6.13.3`