# CVE-2026-81000: net: tun: bound receive headroom

> An integer underflow in the Linux kernel TUN driver allows memory corruption, potentially leading to code execution or denial of service.

- **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:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100232, BELL-CVE-2026-81000, DEBIAN-CVE-2026-81000, GHSA-32hf-vhcc-85c2, UBUNTU-CVE-2026-81000

## What is CVE-2026-81000?

CVE-2026-81000 is a high-severity vulnerability in Linux, published on 11 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: tun: bound receive headroom

tun_get_user() uses tun->align both as skb headroom and when choosing how
much packet data to keep linear. OVS can propagate an oversized headroom
request from another port to TUN or TAP.

When align is larger than the usable space in a one-page skb head,
SKB_MAX_HEAD(align) underflows and the result becomes negative when stored
in good_linear. That value later wraps when assigned to the size_t linear
variable, and tun_alloc_skb() can place skb->data outside the allocated
head.

Bound the headroom stored by TUN to the one-page skb-head budget and the
largest non-sentinel 16-bit skb header offset. Leave one linear byte for
raw TUN and a complete Ethernet header for TAP, including NET_IP_ALIGN.

Also pull the raw-TUN protocol byte and the TAP Ethernet header before
accessing them, so these checks remain safe for nonlinear skbs supplied by
other allocation paths.

## Which versions are affected by CVE-2026-81000?


## How do I fix CVE-2026-81000?

Apply the vendor's patch.

## Is CVE-2026-81000 being exploited?

There is no known exploitation of CVE-2026-81000 in the wild and no public exploit code.

Public exploit code:

- Tracking TUNderflow (CVE-2026-81000), the Linux kernel TUN/TAP receive-headroom underflow (github_poc): https://github.com/suominen/tunderflow
- CVE-2026-81000 (github_poc): https://github.com/0xredash/CVE-2026-81000
- CVE-2026-81000 (github_poc): https://github.com/HORKimhab/CVE-2026-81000

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81000 and fix it if it is affected.

What it is: CVE-2026-81000 is a high-severity vulnerability in Linux, published on 11 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-2026-81000 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-2026-81000/ (Markdown: https://insights.sbomify.com/CVE-2026-81000/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81000
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81000
- OSV: https://osv.dev/vulnerability/CVE-2026-81000

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-81000/
Summaries are generated and may contain mistakes; the official description above is the record.