# CVE-2025-40004: net/9p: Fix buffer overflow in USB transport layer

> A malicious USB host can trigger a heap buffer overflow in the Linux kernel's 9pfs USB transport, potentially leading to code execution or privilege escalation.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 Oct 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-40004, DEBIAN-CVE-2025-40004, GHSA-7p6w-xg84-v7c5, ROOT-OS-DEBIAN-13-CVE-2025-40004, UBUNTU-CVE-2025-40004

## What is CVE-2025-40004?

CVE-2025-40004 is a memory corruption vulnerability in Linux, published on 20 Oct 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/9p: Fix buffer overflow in USB transport layer

A buffer overflow vulnerability exists in the USB 9pfs transport layer
where inconsistent size validation between packet header parsing and
actual data copying allows a malicious USB host to overflow heap buffers.

The issue occurs because:
- usb9pfs_rx_header() validates only the declared size in packet header
- usb9pfs_rx_complete() uses req->actual (actual received bytes) for
memcpy

This allows an attacker to craft packets with small declared size
(bypassing validation) but large actual payload (triggering overflow
in memcpy).

Add validation in usb9pfs_rx_complete() to ensure req->actual does not
exceed the buffer capacity before copying data.

## Which versions are affected by CVE-2025-40004?


## How do I fix CVE-2025-40004?

Apply the vendor's patch.

## Is CVE-2025-40004 being exploited?

There is no known exploitation of CVE-2025-40004 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40004 and fix it if it is affected.

What it is: CVE-2025-40004 is a memory corruption vulnerability in Linux, published on 20 Oct 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-2025-40004 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-2025-40004/ (Markdown: https://insights.sbomify.com/CVE-2025-40004/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40004
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40004
- OSV: https://osv.dev/vulnerability/CVE-2025-40004

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40004/
Summaries are generated and may contain mistakes; the official description above is the record.