CVE-2025-39766 net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit
A logic error in the Linux kernel's CAKE qdisc can trigger a kernel warning and potential denial of service when specific traffic shaping configurations are used.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-39766?
CVE-2025-39766 is a high-severity denial of service vulnerability in Debian Debian Linux, published on 11 Sep 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Which versions are affected by CVE-2025-39766?
It affects 2 products.
Products include Debian Debian Linux, Linux kernel.
How do I fix CVE-2025-39766?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-39766 being exploited?
There is no known exploitation of CVE-2025-39766 in the wild and no public exploit code.
Other vulnerabilities in Debian Debian Linux
Sources
Also known as AZL-67232, AZL-73962, BELL-CVE-2025-39766, DEBIAN-CVE-2025-39766, GHSA-p46x-ph7h-c256, ROOT-OS-DEBIAN-11-CVE-2025-39766 and 5 more.