# CVE-2025-69209: ArduinoCore-avr has Stack-Based Buffer Overflow in WString Float/Double Constructors

> Attacker can trigger stack buffer overflow via high-precision float conversion, potentially enabling code execution or denial of service on Arduino AVR boards.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 Jan 2026
- **Updated:** 21 Jan 2026
- **Also known as:** DEBIAN-CVE-2025-69209, GHSA-pvx3-fm7w-6hjm, UBUNTU-CVE-2025-69209

## What is CVE-2025-69209?

CVE-2025-69209 is a medium-severity memory corruption vulnerability in ArduinoCore-avr, published on 21 Jan 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

ArduinoCore-avr contains the source code and configuration files of the Arduino AVR Boards platform. A vulnerability in versions prior to 1.8.7 allows an attacker to trigger a stack-based buffer overflow when converting floating-point values to strings with high precision. By passing very large `decimalPlaces` values to the affected String constructors or concat methods, the `dtostrf` function writes beyond fixed-size stack buffers, causing memory corruption and denial of service. Under specific conditions, this could enable arbitrary code execution on AVR-based Arduino boards.

### Patches

- The Fix is included starting from the `1.8.7` release available from the following link [ArduinoCore-avr v1.8.7](https://github.com/arduino/ArduinoCore-avr)

- The Fixing Commit is available at the following link [1a6a417f89c8901dad646efce74ae9d3ddebfd59](https://github.com/arduino/ArduinoCore-avr/pull/613/commits/1a6a417f89c8901dad646efce74ae9d3ddebfd59)

### References

- [ASEC-26-001 ArduinoCore-avr vXXXX Resolves Buffer Overflow Vulnerability](https://support.arduino.cc/hc/en-us/articles/XXXXX)

### Credits

- Maxime Rossi Bellom and Ramtine Tofighi Shirazi from SecMate (https://secmate.dev/)

Weakness: CWE-120 (Buffer Copy without Checking Size of Input)

## Which versions are affected by CVE-2025-69209?


## How do I fix CVE-2025-69209?

Upgrade to a fixed version.

## Is CVE-2025-69209 being exploited?

There is no known exploitation of CVE-2025-69209 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-69209 and fix it if it is affected.

What it is: CVE-2025-69209 is a medium-severity memory corruption vulnerability in ArduinoCore-avr, published on 21 Jan 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-2025-69209 in the wild and no public exploit code.

Affected product: ArduinoCore-avr. 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-2025-69209/ (Markdown: https://insights.sbomify.com/CVE-2025-69209/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-69209
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-69209
- OSV: https://osv.dev/vulnerability/CVE-2025-69209

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From sbomify Insights: https://insights.sbomify.com/CVE-2025-69209/
Summaries are generated and may contain mistakes; the official description above is the record.