# CVE-2025-39684: comedi: Fix use of uninitialized memory in do_insn_ioctl() and do_insnlist_ioctl()

> A local attacker can read uninitialized kernel memory via the comedi subsystem, potentially exposing sensitive data.

- **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:** 5 Sep 2025
- **Updated:** 12 May 2026
- **Also known as:** AZL-66953, BELL-CVE-2025-39684, DEBIAN-CVE-2025-39684, GHSA-j349-5p3c-r8v4, ROOT-OS-DEBIAN-11-CVE-2025-39684, ROOT-OS-DEBIAN-12-CVE-2025-39684, ROOT-OS-DEBIAN-13-CVE-2025-39684, ROOT-OS-UBUNTU-2204-CVE-2025-39684, ROOT-OS-UBUNTU-2404-CVE-2025-39684, UBUNTU-CVE-2025-39684

## What is CVE-2025-39684?

CVE-2025-39684 is a medium-severity vulnerability in Debian Debian Linux, published on 5 Sep 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:

comedi: Fix use of uninitialized memory in do_insn_ioctl() and do_insnlist_ioctl()

syzbot reports a KMSAN kernel-infoleak in `do_insn_ioctl()`.  A kernel
buffer is allocated to hold `insn->n` samples (each of which is an
`unsigned int`).  For some instruction types, `insn->n` samples are
copied back to user-space, unless an error code is being returned.  The
problem is that not all the instruction handlers that need to return
data to userspace fill in the whole `insn->n` samples, so that there is
an information leak.  There is a similar syzbot report for
`do_insnlist_ioctl()`, although it does not have a reproducer for it at
the time of writing.

One culprit is `insn_rw_emulate_bits()` which is used as the handler for
`INSN_READ` or `INSN_WRITE` instructions for subdevices that do not have
a specific handler for that instruction, but do have an `INSN_BITS`
handler.  For `INSN_READ` it only fills in at most 1 sample, so if
`insn->n` is greater than 1, the remaining `insn->n - 1` samples copied
to userspace will be uninitialized kernel data.

Another culprit is `vm80xx_ai_insn_read()` in the "vm80xx" driver.  It
never returns an error, even if it fails to fill the buffer.

Fix it in `do_insn_ioctl()` and `do_insnlist_ioctl()` by making sure
that uninitialized parts of the allocated buffer are zeroed before
handling each instruction.

Thanks to Arnaud Lecomte for their fix to `do_insn_ioctl()`.  That fix
replaced the call to `kmalloc_array()` with `kcalloc()`, but it is not
always necessary to clear the whole buffer.

Weakness: CWE-908 (Use of Uninitialized Resource)

## Which versions are affected by CVE-2025-39684?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-39684?

Apply the vendor's patch.

## Is CVE-2025-39684 being exploited?

There is no known exploitation of CVE-2025-39684 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39684 and fix it if it is affected.

What it is: CVE-2025-39684 is a medium-severity vulnerability in Debian Debian Linux, published on 5 Sep 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-2025-39684 in the wild and no public exploit code.

Affected product: Debian Debian 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-2025-39684/ (Markdown: https://insights.sbomify.com/CVE-2025-39684/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39684
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39684
- OSV: https://osv.dev/vulnerability/CVE-2025-39684

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39684/
Summaries are generated and may contain mistakes; the official description above is the record.