# CVE-2026-90109: net: sched: fix 32-bit backlog wrap in gred, bfifo and plug enqueue

> A 32-bit integer overflow in Linux kernel network schedulers allows root users to cause unbounded queue growth and system OOM.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-102656, BELL-CVE-2026-90109, DEBIAN-CVE-2026-90109, GHSA-6qjr-4f2g-wppr, UBUNTU-CVE-2026-90109

## What is CVE-2026-90109?

CVE-2026-90109 is a vulnerability in Linux, published on 17 Sep 2026. 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: fix 32-bit backlog wrap in gred, bfifo and plug enqueue

gred_enqueue(), bfifo_enqueue() and plug_enqueue() admit a packet when the
current backlog plus the packet length fits within the queue limit:

  sch->qstats.backlog + qdisc_pkt_len(skb) <= sch->limit (gred default VQ)
  gred_backlog+qdisc_pkt_len(skb) <= q->limit  (gred configured VQ)
  sch->qstats.backlog + qdisc_pkt_len(skb) <= sch->limit (bfifo)
  sch->qstats.backlog + skb->len <= q->limit             (plug)

sch->qstats.backlog and q->backlog are u32, and qdisc_pkt_len()/skb->len
are unsigned int, so all sums are computed in 32 bits and wrap at 2^32.
Once the true backlog exceeds 4 GiB the wrapped sum becomes small and
admission keeps succeeding, so the queue grows without bound and the kernel
can be driven to OOM.

Promote the sums to u64 so admission stops once the true backlog exceeds
the limit.  The limit is u32, so the bounded queue stays below 2^32 and
the stored u32 backlog never wraps.

The bug can only be reproduced as root (albeit with ridiculous setup):
 attach a gred (or bfifo/plug) qdisc with a limit near 4 GiB,
 leaving the default VQ unconfigured (for gred), and drive >4 GiB of
 queued traffic (e.g. via a size table / stab to inflate qdisc_pkt_len,
 or sustained high-rate traffic). The u32 backlog+len sum wraps at 2^32,
 admission keeps succeeding, and the queue grows unboundedly to OOM.

## Which versions are affected by CVE-2026-90109?


## How do I fix CVE-2026-90109?

Apply the vendor's patch.

## Is CVE-2026-90109 being exploited?

There is no known exploitation of CVE-2026-90109 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90109 and fix it if it is affected.

What it is: CVE-2026-90109 is a vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-90109 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-2026-90109/ (Markdown: https://insights.sbomify.com/CVE-2026-90109/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90109
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90109
- OSV: https://osv.dev/vulnerability/CVE-2026-90109

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90109/
Summaries are generated and may contain mistakes; the official description above is the record.