CVE-2026-74284 net/sched: sch_hfsc: Don't make class passive twice
A logic error in the Linux kernel HFSC scheduler causes network queues to stall, leading to denial of service for remote attackers.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-74284?
CVE-2026-74284 is a denial of service vulnerability in Linux, published on 15 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
net/sched: sch_hfsc: Don't make class passive twice
update_vf() is called from two places for the same class during a single dequeue when the class's child qdisc (e.g. codel/fq_codel) drops its last packets while dequeuing:
1. The child calls qdisc_tree_reduce_backlog(), which, now that the child is empty, invokes hfsc_qlen_notify() -> update_vf(cl, 0, 0) and turns the class passive (cl_nactive is decremented up the hierarchy).
2. hfsc_dequeue() then calls update_vf(cl, qdisc_pkt_len(skb), cur_time) to charge the dequeued bytes.
On the second call the class is already passive, but its child qdisc is still empty, so update_vf() arms go_passive again:
if (cl->qdisc->q.qlen == 0 && cl->cl_flags & HFSC_FSC) go_passive = 1;
The leaf is then skipped by the cl_nactive == 0 check inside the loop, which does not clear go_passive, so the stale go_passive propagates to the parent and decrements its cl_nactive a second time. A parent that still has other active children is driven to cl_nactive == 0 and removed from the vttree, even though those siblings are still backlogged. They are never dequeued again and the qdisc stalls.
Fix this by only arming go_passive when the class is actually active, so an already-passive class no longer triggers a second passive transition. The byte accounting (cl->cl_total += len) still runs for every ancestor, so dequeued bytes continue to be counted exactly once.
Which versions are affected by CVE-2026-74284?
How do I fix CVE-2026-74284?
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-2026-74284 being exploited?
There is no known exploitation of CVE-2026-74284 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-74284, DEBIAN-CVE-2026-74284, GHSA-mfhv-3jvw-9w4g, UBUNTU-CVE-2026-74284.