# CVE-2025-40173: net/ip6_tunnel: Prevent perpetual tunnel growth

> Attackers can cause a denial of service by triggering perpetual headroom growth in IPv6 tunnels, affecting Linux kernel users.

- **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:** 12 Nov 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-70019, BELL-CVE-2025-40173, DEBIAN-CVE-2025-40173, GHSA-hr9g-hq4q-3hfj, ROOT-OS-DEBIAN-11-CVE-2025-40173, ROOT-OS-DEBIAN-13-CVE-2025-40173, ROOT-OS-UBUNTU-2404-CVE-2025-40173, UBUNTU-CVE-2025-40173

## What is CVE-2025-40173?

CVE-2025-40173 is a high-severity denial of service vulnerability in Linux, published on 12 Nov 2025. 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/ip6_tunnel: Prevent perpetual tunnel growth

Similarly to ipv4 tunnel, ipv6 version updates dev->needed_headroom, too.
While ipv4 tunnel headroom adjustment growth was limited in
commit 5ae1e9922bbd ("net: ip_tunnel: prevent perpetual headroom growth"),
ipv6 tunnel yet increases the headroom without any ceiling.

Reflect ipv4 tunnel headroom adjustment limit on ipv6 version.

Credits to Francesco Ruggeri, who was originally debugging this issue
and wrote local Arista-specific patch and a reproducer.

## Which versions are affected by CVE-2025-40173?


## How do I fix CVE-2025-40173?

Apply the vendor's patch.

## Is CVE-2025-40173 being exploited?

There is no known exploitation of CVE-2025-40173 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40173 and fix it if it is affected.

What it is: CVE-2025-40173 is a high-severity denial of service vulnerability in Linux, published on 12 Nov 2025. 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-2025-40173 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-2025-40173/ (Markdown: https://insights.sbomify.com/CVE-2025-40173/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40173
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40173
- OSV: https://osv.dev/vulnerability/CVE-2025-40173

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40173/
Summaries are generated and may contain mistakes; the official description above is the record.