# CVE-2025-40255: net: core: prevent NULL deref in generic_hwtstamp_ioctl_lower()

> A local attacker can crash the Linux kernel by triggering a null pointer dereference in the network timestamping ioctl handler.

- **Severity:** Not scored
- **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:** 4 Dec 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-40255, DEBIAN-CVE-2025-40255, GHSA-3mxh-p32w-7p56, UBUNTU-CVE-2025-40255

## What is CVE-2025-40255?

CVE-2025-40255 is a vulnerability in Linux, published on 4 Dec 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: core: prevent NULL deref in generic_hwtstamp_ioctl_lower()

The ethtool tsconfig Netlink path can trigger a null pointer
dereference. A call chain such as:

  tsconfig_prepare_data() ->
  dev_get_hwtstamp_phylib() ->
  vlan_hwtstamp_get() ->
  generic_hwtstamp_get_lower() ->
  generic_hwtstamp_ioctl_lower()

results in generic_hwtstamp_ioctl_lower() being called with
kernel_cfg->ifr as NULL.

The generic_hwtstamp_ioctl_lower() function does not expect
a NULL ifr and dereferences it, leading to a system crash.

Fix this by adding a NULL check for kernel_cfg->ifr in
generic_hwtstamp_ioctl_lower(). If ifr is NULL, return -EINVAL.

## Which versions are affected by CVE-2025-40255?


## How do I fix CVE-2025-40255?

Apply the vendor's patch.

## Is CVE-2025-40255 being exploited?

There is no known exploitation of CVE-2025-40255 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40255 and fix it if it is affected.

What it is: CVE-2025-40255 is a vulnerability in Linux, published on 4 Dec 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-2025-40255 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-2025-40255/ (Markdown: https://insights.sbomify.com/CVE-2025-40255/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40255
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40255
- OSV: https://osv.dev/vulnerability/CVE-2025-40255

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40255/
Summaries are generated and may contain mistakes; the official description above is the record.