# CVE-2025-40304: fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

> A missing bounds check in the Linux kernel framebuffer driver allows out-of-bounds writes, potentially causing crashes or code execution.

- **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:** 8 Dec 2025
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2025-40304, DEBIAN-CVE-2025-40304, GHSA-6cgq-9674-3xfg, ROOT-OS-DEBIAN-12-CVE-2025-40304, ROOT-OS-DEBIAN-13-CVE-2025-40304, ROOT-OS-UBUNTU-2204-CVE-2025-40304, ROOT-OS-UBUNTU-2404-CVE-2025-40304, UBUNTU-CVE-2025-40304

## What is CVE-2025-40304?

CVE-2025-40304 is a high-severity memory corruption vulnerability in Linux, published on 8 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:

fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

Add bounds checking to prevent writes past framebuffer boundaries when
rendering text near screen edges. Return early if the Y position is off-screen
and clip image height to screen boundary. Break from the rendering loop if the
X position is off-screen. When clipping image width to fit the screen, update
the character count to match the clipped width to prevent buffer size
mismatches.

Without the character count update, bit_putcs_aligned and bit_putcs_unaligned
receive mismatched parameters where the buffer is allocated for the clipped
width but cnt reflects the original larger count, causing out-of-bounds writes.

## Which versions are affected by CVE-2025-40304?


## How do I fix CVE-2025-40304?

Apply the vendor's patch.

## Is CVE-2025-40304 being exploited?

There is no known exploitation of CVE-2025-40304 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40304 and fix it if it is affected.

What it is: CVE-2025-40304 is a high-severity memory corruption vulnerability in Linux, published on 8 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-2025-40304 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-2025-40304/ (Markdown: https://insights.sbomify.com/CVE-2025-40304/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40304
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40304
- OSV: https://osv.dev/vulnerability/CVE-2025-40304

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40304/
Summaries are generated and may contain mistakes; the official description above is the record.