# CVE-2026-80975: mfd: qnap-mcu: keep the reply buffer alive past a command timeout

> A use-after-free vulnerability in the QNAP MCU kernel driver allows attackers to corrupt stack memory via late replies, potentially leading to code execution or privilege escalation.

- **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:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-80975, DEBIAN-CVE-2026-80975, GHSA-r5c2-6w2m-2m2q, UBUNTU-CVE-2026-80975

## What is CVE-2026-80975?

CVE-2026-80975 is a high-severity use after free vulnerability in Linux, published on 11 Sep 2026. 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:

mfd: qnap-mcu: keep the reply buffer alive past a command timeout

qnap_mcu_exec() publishes an on-stack buffer to the receive path:

	unsigned char rx[QNAP_MCU_RX_BUFFER_SIZE];
	...
	reply->data = rx;
	reply->length = length;

and qnap_mcu_receive_buf() writes into it from the serdev receive path,
which runs out of flush_to_ldisc() and is not serialized against
qnap_mcu_exec() at all. bus_lock cannot cover it, because qnap_mcu_exec()
holds that mutex across wait_for_completion_timeout().

On a timeout qnap_mcu_exec() returns with reply->data still pointing at
its own frame. A reply that arrives late, or an unsolicited message from
the MCU, is then written into a stack frame that has been left, corrupting
whatever runs next on that stack. The same applies when qnap_mcu_write()
fails, since that path returns without touching the reply state either.

Move the receive buffer into struct qnap_mcu. It is 37 bytes and the
structure is devm_kzalloc()ed, so it lives as long as the driver, and a
late write lands in memory that is still valid and is reinitialized by the
next command. bus_lock keeps commands from sharing it.

This deliberately does not clear reply->data or reply->length on the
timeout path. Doing so races with qnap_mcu_receive_buf(), which reads both
after its

	if (!reply->length)
		return size;

check: clearing reply->data gives a NULL dereference, and clearing
reply->length alone removes the reply->received == reply->length exit
condition, so the copy loop runs until the uart chunk is consumed and
overruns the buffer. Leaving both set keeps the write bounded by
reply->length, which qnap_mcu_exec() has already checked against
sizeof(mcu->rx).

## Which versions are affected by CVE-2026-80975?


## How do I fix CVE-2026-80975?

Apply the vendor's patch.

## Is CVE-2026-80975 being exploited?

There is no known exploitation of CVE-2026-80975 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80975 and fix it if it is affected.

What it is: CVE-2026-80975 is a high-severity use after free vulnerability in Linux, published on 11 Sep 2026. 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-2026-80975 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-2026-80975/ (Markdown: https://insights.sbomify.com/CVE-2026-80975/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80975
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80975
- OSV: https://osv.dev/vulnerability/CVE-2026-80975

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80975/
Summaries are generated and may contain mistakes; the official description above is the record.