# CVE-2026-90075: net/sched: fq_codel: clamp default quantum and mtu

> A local attacker with CAP_NET_ADMIN can cause a kernel soft lockup via integer overflow in fq_codel when configuring a device with an extremely large MTU.

- **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:** 19 Oct 2026
- **Also known as:** AZL-101846, BELL-CVE-2026-90075, DEBIAN-CVE-2026-90075, GHSA-cgq5-rq9f-qphv, UBUNTU-CVE-2026-90075

## What is CVE-2026-90075?

CVE-2026-90075 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: fq_codel: clamp default quantum and mtu

fq_codel_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) without
clamping. A device with a huge MTU (e.g. dummy with max_mtu == 0
accepting MTU 2147483634) makes psched_mtu() return 0x80000000, which
overflows the signed flow->deficit to INT_MIN in fq_codel_dequeue(),
causing an infinite loop and soft lockup. Emulate fq_codel_change()
and constrain to [256, FQ_CODEL_QUANTUM_MAX].

The same unclamped psched_mtu() is assigned to q->cparams.mtu a bit
below, and fq_codel_change() never updates it. codel_should_drop()
tests "*backlog <= params->mtu"; with mtu == 0x80000000 (~2 GiB) and
the default 32 MiB memory_limit, the test is always true, so CoDel is
silently and completely disabled (no drops, no ECN). Declare a single
clamped mtu and assign both q->quantum and q->cparams.mtu from it,
which also removes the double psched_mtu() call.

Conditions to recreate the bug: a device whose MTU (plus
hard_header_len) wraps psched_mtu() into the sign bit (e.g. a dummy
device with max_mtu == 0 accepting MTU 2147483634). Requires
CAP_NET_ADMIN in a user namespace.

## Which versions are affected by CVE-2026-90075?


## How do I fix CVE-2026-90075?

Apply the vendor's patch.

## Is CVE-2026-90075 being exploited?

There is no known exploitation of CVE-2026-90075 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90075 and fix it if it is affected.

What it is: CVE-2026-90075 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-2026-90075 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-2026-90075/ (Markdown: https://insights.sbomify.com/CVE-2026-90075/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90075
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90075
- OSV: https://osv.dev/vulnerability/CVE-2026-90075

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90075/
Summaries are generated and may contain mistakes; the official description above is the record.