# CVE-2025-40363: net: ipv6: fix field-spanning memcpy warning in AH output

> A memory corruption issue in the Linux kernel IPv6 AH output path can cause a kernel warning and potential denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Dec 2025
- **Updated:** 16 Jun 2026
- **Also known as:** BELL-CVE-2025-40363, DEBIAN-CVE-2025-40363, GHSA-9gfg-vfjm-p94q, ROOT-OS-DEBIAN-12-CVE-2025-40363, ROOT-OS-DEBIAN-13-CVE-2025-40363, ROOT-OS-UBUNTU-2204-CVE-2025-40363, ROOT-OS-UBUNTU-2404-CVE-2025-40363, UBUNTU-CVE-2025-40363

## What is CVE-2025-40363?

CVE-2025-40363 is a memory corruption vulnerability in Linux, published on 16 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

net: ipv6: fix field-spanning memcpy warning in AH output

Fix field-spanning memcpy warnings in ah6_output() and
ah6_output_done() where extension headers are copied to/from IPv6
address fields, triggering fortify-string warnings about writes beyond
the 16-byte address fields.

  memcpy: detected field-spanning write (size 40) of single field "&top_iph->saddr" at net/ipv6/ah6.c:439 (size 16)
  WARNING: CPU: 0 PID: 8838 at net/ipv6/ah6.c:439 ah6_output+0xe7e/0x14e0 net/ipv6/ah6.c:439

The warnings are false positives as the extension headers are
intentionally placed after the IPv6 header in memory. Fix by properly
copying addresses and extension headers separately, and introduce
helper functions to avoid code duplication.

## Which versions are affected by CVE-2025-40363?


## How do I fix CVE-2025-40363?

Apply the vendor's patch.

## Is CVE-2025-40363 being exploited?

There is no known exploitation of CVE-2025-40363 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40363 and fix it if it is affected.

What it is: CVE-2025-40363 is a memory corruption vulnerability in Linux, published on 16 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-40363 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-2025-40363/ (Markdown: https://insights.sbomify.com/CVE-2025-40363/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40363
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40363
- OSV: https://osv.dev/vulnerability/CVE-2025-40363

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40363/
Summaries are generated and may contain mistakes; the official description above is the record.