CVE-2025-39766
Description
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.
Predictions
Heuristic predictions, AS-IS, for prioritization only.
Mitigations
No mitigations published for this CVE yet.
The vendor-content worker queues fetches as references arrive (check back in a few minutes). Or — if you've already worked around this in production — publish your fix to the community-verified tier.
✚ Propose a mitigation on Community → Mitigations published via the community go through AI scoring + 2 human reviewers + 7-day silent objection window before landing here withsource_tier=community-verified.
OS impact
Linux kernel Affected 2 releases
| Version | Status | Fixed in |
|---|---|---|
| 6.17 | Affected | — |
| — | Affected | 5.4.297 |
SUSE Affected 1 release
| Version | Status | Fixed in |
|---|---|---|
| — | Affected | — |
Debian Mixed 6 releases
| Version | Status | Fixed in |
|---|---|---|
| trixie | Fixed | 6.12.48-1 |
| sid | Fixed | 6.16.5-1 |
| forky | Fixed | 6.16.5-1 |
| bullseye | Fixed | 5.10.244-1 |
| bookworm | Fixed | 6.1.153-1 |
| 11.0 | Affected | — |
AlmaLinux Fixed 1 release
| Version | Status | Fixed in |
|---|---|---|
| 9 | Fixed | kernel-headers-5.14.0-611.49.1.el9_7.aarch64.rpm |
Red Hat Fixed 1 release
| Version | Status | Fixed in |
|---|---|---|
| 9 | Fixed | — |
Rocky Linux Fixed 1 release
| Version | Status | Fixed in |
|---|---|---|
| 9 | Fixed | — |
References
- https://access.redhat.com/errata/RHSA-2026:19568
- https://access.redhat.com/errata/RHSA-2026:8921
- https://git.kernel.org/stable/c/0dacfc5372e314d1219f03e64dde3ab495a5a25e
- https://git.kernel.org/stable/c/15de71d06a400f7fdc15bf377a2552b0ec437cf5
- https://git.kernel.org/stable/c/62d591dde4defb1333d202410609c4ddeae060b3
- https://git.kernel.org/stable/c/710866fc0a64eafcb8bacd91bcb1329eb7e5035f
- https://git.kernel.org/stable/c/7689ab22de36f8db19095f6bdf11f28cfde92f5c
- https://git.kernel.org/stable/c/aa12ee1c1bd260943fd6ab556d8635811c332eeb
- https://git.kernel.org/stable/c/de04ddd2980b48caa8d7e24a7db2742917a8b280
- https://git.kernel.org/stable/c/ff57186b2cc39766672c4c0332323933e5faaa88
- https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html
- https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html
- https://cert-portal.siemens.com/productcert/html/ssa-032379.html
- https://www.suse.com/security/cve/CVE-2025-39766.html
- https://security-tracker.debian.org/tracker/CVE-2025-39766
- https://bugzilla.redhat.com/2394648
- https://bugzilla.redhat.com/2425046
- https://errata.almalinux.org/9/ALSA-2026-8921.html
- https://errata.rockylinux.org/RLSA-2026:8921
Community-verified mitigations for this CVE will appear above when contributors publish them.
Verify integrity in audit chain (admin only). AS-IS.