# 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.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 17 Aug 2026
- **Also known as:** BELL-CVE-2026-74284, DEBIAN-CVE-2026-74284, GHSA-mfhv-3jvw-9w4g, UBUNTU-CVE-2026-74284

## 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.

## Is CVE-2026-74284 being exploited?

There is no known exploitation of CVE-2026-74284 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74284 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-74284 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-74284/ (Markdown: https://insights.sbomify.com/CVE-2026-74284/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74284
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74284
- OSV: https://osv.dev/vulnerability/CVE-2026-74284

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74284/
Summaries are generated and may contain mistakes; the official description above is the record.