# CVE-2021-3966: Usb bluetooth device ACL read cb buffer overflow

> A buffer overflow in the Zephyr RTOS USB Bluetooth driver allows attackers to execute arbitrary code or cause a denial of service via malicious USB devices.

- **Severity:** Critical 9.6 (`CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 11 Jan 2023
- **Updated:** 9 Apr 2025

## What is CVE-2021-3966?

CVE-2021-3966 is a critical memory corruption vulnerability in Zephyrproject Zephyr, published on 11 Jan 2023. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

usb device bluetooth class includes a buffer overflow related to implementation of net_buf_add_mem.

Weakness: CWE-120 (Buffer Copy without Checking Size of Input), CWE-122 (Heap-based Buffer Overflow)

## Which versions are affected by CVE-2021-3966?

It affects 1 product.


Products: Zephyrproject Zephyr.

## How do I fix CVE-2021-3966?

No fix has been published for CVE-2021-3966 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-3966 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-3966 exists.

## Other vulnerabilities in Zephyrproject Zephyr

- [CVE-2026-5067](https://insights.sbomify.com/CVE-2026-5067/): Out-of-bounds read/write in HTTP WebSocket upgrade via non-null-terminated Sec-WebSocket-Key (critical 9.8)
- [CVE-2022-3806](https://insights.sbomify.com/CVE-2022-3806/): Bluetooth HCI Error Handling Double Free (critical 9.8)
- [CVE-2017-14199](https://insights.sbomify.com/CVE-2017-14199/): Memory corruption in getaddrinfo (critical 9.8)
- [CVE-2018-1000800](https://insights.sbomify.com/CVE-2018-1000800/): Null dereference in n/a (critical 9.8)
- [CVE-2021-3329](https://insights.sbomify.com/CVE-2021-3329/): DOS: Incorrect handling of the initial HCI ACL_MTU handshake packet leads to crash of bluetooth host layer (critical 9.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-3966 and fix it if it is affected.

What it is: CVE-2021-3966 is a critical memory corruption vulnerability in Zephyrproject Zephyr, published on 11 Jan 2023. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-3966 exists.

Affected product: Zephyrproject Zephyr. 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-2021-3966/ (Markdown: https://insights.sbomify.com/CVE-2021-3966/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3966
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3966
- OSV: https://osv.dev/vulnerability/CVE-2021-3966

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3966/
Summaries are generated and may contain mistakes; the official description above is the record.