# CVE-2026-74544: net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds

> An unprivileged user can trigger undefined behavior and potential denial of service in the Linux kernel by providing an invalid offshift value to the cls_u32 classifier.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-96395, BELL-CVE-2026-74544, DEBIAN-CVE-2026-74544, GHSA-p37c-4w2w-mm9f, UBUNTU-CVE-2026-74544

## What is CVE-2026-74544?

CVE-2026-74544 is a high-severity vulnerability in Linux, published on 15 Aug 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: cls_u32: validate offshift to prevent shift-out-of-bounds

u32_change() copies the user-provided tc_u32_sel.offshift (unsigned char,
0-255) into the kernel knode object without bounds validation. When a
packet later hits u32_classify() with TC_U32_VAROFFSET set, it evaluates
`ntohs(offmask & *data) >> offshift` where the left operand is a 16-bit
value promoted to a 32-bit int. Any offshift >= 32 is undefined behavior
per C11 6.5.7p3, triggerable by an unprivileged user via user/network
namespaces.

UBSAN: shift-out-of-bounds in net/sched/cls_u32.c:236:43
shift exponent 32 is too large for 32-bit type int

Fix this by rejecting offshift >= 16 during filter creation in
u32_change().

## Which versions are affected by CVE-2026-74544?


## How do I fix CVE-2026-74544?

Apply the vendor's patch.

## Is CVE-2026-74544 being exploited?

There is no known exploitation of CVE-2026-74544 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74544 and fix it if it is affected.

What it is: CVE-2026-74544 is a high-severity vulnerability in Linux, published on 15 Aug 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-2026-74544 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-2026-74544/ (Markdown: https://insights.sbomify.com/CVE-2026-74544/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74544
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74544
- OSV: https://osv.dev/vulnerability/CVE-2026-74544

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74544/
Summaries are generated and may contain mistakes; the official description above is the record.