CVE-2025-69209: 
Linux Debian vulnerability analysis and mitigation

CVE-2025-69209 is a stack-based buffer overflow vulnerability in the WString float/double constructors of ArduinoCore-avr, the source code and configuration platform for Arduino AVR Boards. It affects all versions of ArduinoCore-avr up to and including 1.8.6, and was disclosed on January 21, 2026. The vulnerability was discovered by Maxime Rossi Bellom and Ramtine Tofighi Shirazi from SecMate. It carries a CVSS v4.0 base score of 6.9 (Medium) (GitHub Advisory, Feedly).

The root cause is a classic buffer overflow (CWE-120) in cores/arduino/WString.cpp, where the String constructors for float and double types allocated a fixed 33-byte stack buffer (char buf[33]) before passing it to the dtostrf function. An attacker or malicious sketch can pass an excessively large decimalPlaces argument to these constructors or related concat methods, causing dtostrf to write beyond the fixed buffer boundary and corrupt adjacent stack memory. The fix in version 1.8.7 replaces the hardcoded buffer with a dynamically sized buffer based on FLT_MAX_10_EXP/DBL_MAX_10_EXP constants and clamps the decimalPlaces parameter to a safe maximum before calling dtostrf (GitHub Advisory, Fix Commit).

Successful exploitation causes memory corruption on the AVR microcontroller's stack, leading to denial of service (device crash or hang) as the primary impact. Under specific conditions — depending on the sketch's memory layout and the attacker's ability to control the overflow content — arbitrary code execution on the AVR-based Arduino board may be achievable. Because AVR boards are commonly used in embedded and IoT applications, exploitation could disrupt physical processes or safety-critical functions controlled by the device (GitHub Advisory, Feedly).

No public proof-of-concept exploit code or in-the-wild exploitation has been reported for CVE-2025-69209. The attack vector is local (the vulnerability is triggered by code running on the device, such as a malicious or untrusted Arduino sketch), which significantly limits remote exploitability. The EPSS score is approximately 0.016%, indicating a very low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog (Feedly).

1. Identify a vulnerable target: Confirm the target Arduino AVR board is running a sketch compiled with ArduinoCore-avr version 1.8.6 or earlier.
2. Craft a malicious sketch or input: Write or inject Arduino sketch code that calls a String constructor or concat method with a floating-point value and an excessively large decimalPlaces argument (e.g., String(1.0f, 200)).
3. Trigger the overflow: When the sketch executes, the String(float, unsigned char decimalPlaces) constructor calls dtostrf with the oversized decimalPlaces value, writing beyond the 33-byte stack buffer and corrupting adjacent stack memory.
4. Achieve denial of service or code execution: The stack corruption causes the device to crash (denial of service). Under specific memory layout conditions, a carefully crafted overflow could overwrite the return address or function pointers to redirect execution flow and achieve arbitrary code execution on the AVR microcontroller (GitHub Advisory, Fix Commit).

• Device Behavior: Unexpected resets, hangs, or erratic behavior on AVR-based Arduino boards when processing floating-point-to-string conversions.
• Code Review: Presence of Arduino sketch code calling String(floatValue, largeDecimalPlaces) or String(doubleValue, largeDecimalPlaces) with decimalPlaces values exceeding typical precision needs (e.g., > 10).
• Firmware Analysis: Static analysis of compiled sketch binaries revealing calls to dtostrf with unbounded precision parameters in WString.cpp from ArduinoCore-avr versions ≤ 1.8.6.

The primary remediation is to update ArduinoCore-avr to version 1.8.7 or later, which clamps the decimalPlaces parameter and uses a correctly sized stack buffer. Users can update via the Arduino IDE or Arduino CLI by upgrading the "Arduino AVR Boards" platform package. No configuration-based workaround is available; the fix requires a code-level change. Developers should also audit any custom sketches that pass user-controlled values as decimalPlaces to String constructors (GitHub Release, Arduino Advisory).

Arduino published an official security advisory (ASEC-26-001) acknowledging the vulnerability and crediting the SecMate researchers. Red Hat also tracked the CVE in their security database. Community reaction has been limited given the embedded/IoT scope of the affected platform, with no significant broader media coverage identified (GitHub Advisory, Red Hat CVE).