# CVE-2025-21840: thermal/netlink: Prevent userspace segmentation fault by adjusting UAPI header

> A kernel thermal netlink header change caused userspace segmentation faults in tools like intel-lpmd, leading to denial of service.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 7 Mar 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-21840, GHSA-pcf3-xr6w-mj6g, UBUNTU-CVE-2025-21840

## What is CVE-2025-21840?

CVE-2025-21840 is a medium-severity vulnerability in Linux kernel, published on 7 Mar 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:

thermal/netlink: Prevent userspace segmentation fault by adjusting UAPI header

The intel-lpmd tool [1], which uses the THERMAL_GENL_ATTR_CPU_CAPABILITY
attribute to receive HFI events from kernel space, encounters a
segmentation fault after commit 1773572863c4 ("thermal: netlink: Add the
commands and the events for the thresholds").

The issue arises because the THERMAL_GENL_ATTR_CPU_CAPABILITY raw value
was changed while intel_lpmd still uses the old value.

Although intel_lpmd can be updated to check the THERMAL_GENL_VERSION and
use the appropriate THERMAL_GENL_ATTR_CPU_CAPABILITY value, the commit
itself is questionable.

The commit introduced a new element in the middle of enum thermal_genl_attr,
which affects many existing attributes and introduces potential risks
and unnecessary maintenance burdens for userspace thermal netlink event
users.

Solve the issue by moving the newly introduced
THERMAL_GENL_ATTR_TZ_PREV_TEMP attribute to the end of the
enum thermal_genl_attr. This ensures that all existing thermal generic
netlink attributes remain unaffected.

[ rjw: Subject edits ]

## Which versions are affected by CVE-2025-21840?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21840?

Apply the vendor's patch.

## Is CVE-2025-21840 being exploited?

There is no known exploitation of CVE-2025-21840 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-21840 and fix it if it is affected.

What it is: CVE-2025-21840 is a medium-severity vulnerability in Linux kernel, published on 7 Mar 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-2025-21840 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2025-21840/ (Markdown: https://insights.sbomify.com/CVE-2025-21840/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21840
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21840
- OSV: https://osv.dev/vulnerability/CVE-2025-21840

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21840/
Summaries are generated and may contain mistakes; the official description above is the record.