CVE-2025-21703: linux kernel Use-After-Free — Fix

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.

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CVSS v3.1 Score

7.8

HIGH

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Weakness Type (CWE)

CWE-416 Use After Free

Affected Products

Vendor	Product	Versions
linux	linux_kernel	5.4.288 — 5.4.291
linux	linux_kernel	5.10.232 — 5.10.235
linux	linux_kernel	5.15.175 — 5.15.179
linux	linux_kernel	6.1.121 — 6.1.129
linux	linux_kernel	6.6.67 — 6.6.78
linux	linux_kernel	6.12.6 — 6.12.14
linux	linux_kernel	6.13 — 6.13.3
linux	linux_kernel	All

References

Advisories & Patches

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

Other References

https://lists.debian.org/debian-lts-announce/2025/03/msg00028.html

Frequently Asked Questions

What is CVE-2025-21703? +

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. It has a CVSS v3.1 base score of 7.8 (HIGH).

How severe is CVE-2025-21703? +

CVE-2025-21703 has a CVSS v3.1 score of 7.8 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

What products are affected by CVE-2025-21703? +

CVE-2025-21703 affects products from linux, specifically: linux_kernel. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-21703? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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