# CVE-2026-90076: net/sched: fq: add overflow bounds to quantum and initial quantum

> A local attacker with CAP_NET_ADMIN can cause a kernel soft lockup by configuring a network device with an excessively large MTU, leading to an integer overflow in the fq scheduler.

- **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-102791, BELL-CVE-2026-90076, DEBIAN-CVE-2026-90076, GHSA-6j38-2pg6-m9mv, UBUNTU-CVE-2026-90076

## What is CVE-2026-90076?

CVE-2026-90076 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: add overflow bounds to quantum and initial quantum

fq_init() computes quantum = 2 * psched_mtu() and initial_quantum = 10 *
psched_mtu() with no overflow check. A device with a huge MTU (e.g. dummy
with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return
0x80000000; the 2 * and 10 * multiplications wrap to 0 in 32-bit
arithmetic, so q->quantum == 0. Then in fq_dequeue() the credit-refill
loop adds 0 to f->credit (which stays <= 0) and goto begin loops
forever under the qdisc lock, creating a soft lockup.

Clamp psched_mtu() to [1, 1 << 20] before multiplying so the product
cannot wrap, then cap the result at 1 << 20, matching the bound already
enforced on TCA_FQ_QUANTUM in fq_change().

Conditions to recreate the bug: a device whose MTU (plus
hard_header_len) is large enough that 2 * psched_mtu() wraps (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-2026-90076?


## How do I fix CVE-2026-90076?

Apply the vendor's patch.

## Is CVE-2026-90076 being exploited?

There is no known exploitation of CVE-2026-90076 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90076 and fix it if it is affected.

What it is: CVE-2026-90076 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-2026-90076 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-2026-90076/ (Markdown: https://insights.sbomify.com/CVE-2026-90076/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90076
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90076
- OSV: https://osv.dev/vulnerability/CVE-2026-90076

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90076/
Summaries are generated and may contain mistakes; the official description above is the record.