# CVE-2025-21704: usb: cdc-acm: Check control transfer buffer size before access

> A memory corruption vulnerability in the Linux kernel's cdc-acm driver allows attackers to potentially execute code or escalate privileges via crafted USB notifications.

- **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:** 22 Feb 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2025-21704, DEBIAN-CVE-2025-21704, GHSA-48g3-w2gf-xjrv, ROOT-OS-UBUNTU-2404-CVE-2025-21704, UBUNTU-CVE-2025-21704

## What is CVE-2025-21704?

CVE-2025-21704 is a high-severity memory corruption vulnerability in Linux kernel, published on 22 Feb 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:

usb: cdc-acm: Check control transfer buffer size before access

If the first fragment is shorter than struct usb_cdc_notification, we can't
calculate an expected_size. Log an error and discard the notification
instead of reading lengths from memory outside the received data, which can
lead to memory corruption when the expected_size decreases between
fragments, causing `expected_size - acm->nb_index` to wrap.

This issue has been present since the beginning of git history; however,
it only leads to memory corruption since commit ea2583529cd1
("cdc-acm: reassemble fragmented notifications").

A mitigating factor is that acm_ctrl_irq() can only execute after userspace
has opened /dev/ttyACM*; but if ModemManager is running, ModemManager will
do that automatically depending on the USB device's vendor/product IDs and
its other interfaces.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2025-21704?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21704?

Apply the vendor's patch.

## Is CVE-2025-21704 being exploited?

There is no known exploitation of CVE-2025-21704 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-21704 and fix it if it is affected.

What it is: CVE-2025-21704 is a high-severity memory corruption vulnerability in Linux kernel, published on 22 Feb 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-2025-21704 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2025-21704/ (Markdown: https://insights.sbomify.com/CVE-2025-21704/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21704
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21704
- OSV: https://osv.dev/vulnerability/CVE-2025-21704

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21704/
Summaries are generated and may contain mistakes; the official description above is the record.