# CVE-2025-39686: comedi: Make insn_rw_emulate_bits() do insn->n samples

> A local attacker can read uninitialized kernel memory via the comedi subsystem's bit emulation handler, leading to information disclosure.

- **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:** 5 Sep 2025
- **Updated:** 12 May 2026
- **Also known as:** AZL-66938, BELL-CVE-2025-39686, DEBIAN-CVE-2025-39686, GHSA-v7fr-3jhm-3qhw, ROOT-OS-DEBIAN-11-CVE-2025-39686, ROOT-OS-DEBIAN-12-CVE-2025-39686, ROOT-OS-DEBIAN-13-CVE-2025-39686, ROOT-OS-UBUNTU-2204-CVE-2025-39686, ROOT-OS-UBUNTU-2404-CVE-2025-39686, UBUNTU-CVE-2025-39686

## What is CVE-2025-39686?

CVE-2025-39686 is a high-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: Make insn_rw_emulate_bits() do insn->n samples

The `insn_rw_emulate_bits()` function is used as a default handler for
`INSN_READ` instructions for subdevices that have a handler for
`INSN_BITS` but not for `INSN_READ`.  Similarly, it is used as a default
handler for `INSN_WRITE` instructions for subdevices that have a handler
for `INSN_BITS` but not for `INSN_WRITE`. It works by emulating the
`INSN_READ` or `INSN_WRITE` instruction handling with a constructed
`INSN_BITS` instruction.  However, `INSN_READ` and `INSN_WRITE`
instructions are supposed to be able read or write multiple samples,
indicated by the `insn->n` value, but `insn_rw_emulate_bits()` currently
only handles a single sample.  For `INSN_READ`, the comedi core will
copy `insn->n` samples back to user-space.  (That triggered KASAN
kernel-infoleak errors when `insn->n` was greater than 1, but that is
being fixed more generally elsewhere in the comedi core.)

Make `insn_rw_emulate_bits()` either handle `insn->n` samples, or return
an error, to conform to the general expectation for `INSN_READ` and
`INSN_WRITE` handlers.

## Which versions are affected by CVE-2025-39686?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-39686?

Apply the vendor's patch.

## Is CVE-2025-39686 being exploited?

There is no known exploitation of CVE-2025-39686 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-2025-39686 and fix it if it is affected.

What it is: CVE-2025-39686 is a high-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-2025-39686 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-2025-39686/ (Markdown: https://insights.sbomify.com/CVE-2025-39686/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39686
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39686
- OSV: https://osv.dev/vulnerability/CVE-2025-39686

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39686/
Summaries are generated and may contain mistakes; the official description above is the record.