CVE-2025-39766

High

In the Linux kernel, the following vulnerability has been resolved: net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit The following setup can trigger a WARNING in htb_activate due to the condition: !cl->leaf.q->q.qlen tc qdisc del dev lo root tc qdisc add dev lo root handle 1: htb default 1 tc class add dev lo parent 1: classid 1:1 \ htb rate 64bit tc qdisc add dev lo parent 1:1 handle f: \ cake memlimit 1b ping -I lo -f -c1 -s64 -W0.001 127.0.0.1 This is because the low memlimit leads to a low buffer_limit, which causes packet dropping. However, cake_enqueue still returns NET_XMIT_SUCCESS, causing htb_enqueue to call htb_activate with an empty child qdisc. We should return NET_XMIT_CN when packets are dropped from the same tin and flow. I do not believe return value of NET_XMIT_CN is necessary for packet drops in the case of ack filtering, as that is meant to optimize performance, not to signal congestion.

Package Linux Kernel
Published 2025-09-11
Last modified 2026-05-12
CVSS version 3.1
Patch available
Yes

CVSS 3.1 score

7.8

out of 10
High
Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High
Vector string
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Affected versions

Linux kernel versions 4.19 and later are affected. Fixed in 5.4.297, 5.10.241, 5.15.190, 6.1.149, 6.6.103, 6.12.44, 6.16.4, 6.17 and their respective stable series.

Affected from
≥ 4.19
Fixed in
✓ 5.4.297 5.4.x ✓ 5.10.241 5.10.x ✓ 5.15.190 5.15.x ✓ 6.1.149 6.1.x ✓ 6.6.103 6.6.x ✓ 6.12.44 6.12.x ✓ 6.16.4 6.16.x ✓ 6.17

References

The following references provide additional information about CVE-2025-39766 including vendor advisories, patch commits, exploit details, and third-party analysis. Links are sourced from the NIST NVD database.

8 patch commits total View all on NVD

Frequently asked questions

  • What is CVE-2025-39766?

    CVE-2025-39766 is a High severity Linux kernel vulnerability with a CVSS score of 7.8 out of 10 . It affects Linux kernel versions from 4.19 onward and has been patched in 5.4.297, 5.10.241, 5.15.190 and others. CVE-2025-39766 has not been confirmed as actively exploited and is not listed in the CISA KEV catalog.

  • What is the CVSS score for CVE-2025-39766?

    CVE-2025-39766 has a CVSS score of 7.8 out of 10, rated High severity (CVSS 3.1). The vector string is CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H .

  • Is there a patch available for CVE-2025-39766?

    Yes — CVE-2025-39766 has been patched. Fixed versions include 5.4.297, 5.10.241, 5.15.190 and others. If you are running Linux kernel 4.19 or later up to the fix versions, apply the relevant patch for your kernel branch.

  • Is CVE-2025-39766 actively exploited?

    No — CVE-2025-39766 has not been confirmed as actively exploited. It is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

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