CVE-2025-69209: ArduinoCore-avr Buffer Overflow Flaw

CVE-2025-69209 Overview

CVE-2025-69209 is a stack-based buffer overflow vulnerability affecting ArduinoCore-avr, the source code and configuration files platform for Arduino AVR Boards. The vulnerability exists in versions prior to 1.8.7 and allows an attacker to trigger memory corruption 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 denial of service. Under specific conditions, this vulnerability could enable arbitrary code execution on AVR-based Arduino boards.

Critical Impact

Stack-based buffer overflow in floating-point to string conversion allows memory corruption and denial of service, with potential for arbitrary code execution on AVR-based Arduino hardware.

Affected Products

• ArduinoCore-avr versions prior to 1.8.7
• Arduino AVR Boards using vulnerable ArduinoCore-avr library
• IoT devices and embedded systems running unpatched Arduino AVR firmware

Discovery Timeline

• 2026-01-21 - CVE CVE-2025-69209 published to NVD
• 2026-01-21 - Last updated in NVD database

Technical Details for CVE-2025-69209

Vulnerability Analysis

This vulnerability is classified under CWE-120 (Buffer Copy without Checking Size of Input), a classic memory safety issue that occurs when data is copied to a buffer without proper bounds checking. The flaw resides in the String class implementation within the ArduinoCore-avr library, specifically in how floating-point values are converted to string representations.

The vulnerable code path involves the String constructors and concat methods that accept floating-point numbers along with a precision parameter (decimalPlaces). When an excessively large precision value is provided, the underlying dtostrf function attempts to write more characters than the fixed-size stack buffer can accommodate. Since AVR microcontrollers operate with limited memory and lack sophisticated memory protection mechanisms, this overflow directly corrupts adjacent stack memory, leading to unpredictable behavior including crashes and potential control flow hijacking.

The local attack vector requires an attacker to either have direct access to upload malicious firmware or influence input parameters that are passed to the vulnerable functions. While exploitation complexity is low, the impact is primarily on availability, causing denial of service through memory corruption and system instability.

Root Cause

The root cause lies in the absence of input validation on the decimalPlaces parameter before passing it to the dtostrf function. The original implementation assumed that callers would provide reasonable precision values, but failed to enforce maximum bounds. The dtostrf function, which converts double-precision floating-point numbers to ASCII strings, writes directly to a stack-allocated buffer without checking if the requested precision would exceed buffer capacity.

Attack Vector

The attack requires local access to the Arduino system, either through firmware upload capabilities or by exploiting application logic that passes user-controlled data to the vulnerable String functions. An attacker would craft input that specifies an unreasonably large decimalPlaces value when constructing String objects from floating-point numbers or when using concatenation methods that perform float-to-string conversion.

The attack flow proceeds as follows: malicious input specifying high precision values is passed to String() constructors or concat() methods that handle float/double types, which in turn invoke dtostrf with the attacker-controlled precision, causing stack buffer overflow and memory corruption.

// Security patch in cores/arduino/WString.cpp - Merge pull request #613 from pennam/string-fix
 */
 
 #include "WString.h"
+#include <float.h>
+
+/*********************************************/
+/*  Static Member Initialisation             */
+/*********************************************/
+
+size_t const String::FLT_MAX_DECIMAL_PLACES = DECIMAL_DIG;
+size_t const String::DBL_MAX_DECIMAL_PLACES = DECIMAL_DIG;
 
 /*********************************************/
 /*  Constructors                             */

Source: GitHub Commit

// Security patch in cores/arduino/WString.h - Merge pull request #613 from pennam/string-fix
 	typedef void (String::*StringIfHelperType)() const;
 	void StringIfHelper() const {}
 
+	static size_t const FLT_MAX_DECIMAL_PLACES;
+	static size_t const DBL_MAX_DECIMAL_PLACES;
+
 public:
 	// constructors
 	// creates a copy of the initial value.

Source: GitHub Commit

Detection Methods for CVE-2025-69209

Indicators of Compromise

• Unexpected device crashes or reboots when processing floating-point data
• Memory corruption errors in Arduino system logs or serial output
• Abnormal stack pointer values or corrupted return addresses during debugging
• Devices entering undefined states when handling numeric input with high precision values

Detection Strategies

• Review firmware source code for String constructor calls with float/double parameters where precision values are derived from external input
• Implement static analysis to identify unchecked decimalPlaces parameters in String operations
• Monitor device behavior for unexpected crashes correlating with float-to-string conversion operations
• Conduct code audits focusing on boundary validation for numeric precision parameters

Monitoring Recommendations

• Enable serial debugging output to capture memory corruption events on development boards
• Implement watchdog timers to detect and recover from crash conditions caused by buffer overflows
• Log all firmware upload activities and validate firmware integrity before deployment
• Monitor IoT fleet devices for unusual reboot patterns that may indicate exploitation attempts

How to Mitigate CVE-2025-69209

Immediate Actions Required

• Upgrade ArduinoCore-avr to version 1.8.7 or later immediately
• Review existing firmware code for vulnerable float-to-string conversion patterns
• Validate all external inputs before passing precision values to String constructors
• Deploy updated firmware to all affected Arduino AVR-based devices

Patch Information

Arduino has released version 1.8.7 of ArduinoCore-avr which addresses this vulnerability. The fix introduces static constants FLT_MAX_DECIMAL_PLACES and DBL_MAX_DECIMAL_PLACES based on the DECIMAL_DIG macro from <float.h>, which enforces maximum bounds on precision values before they reach the dtostrf function.

The patched version is available from the ArduinoCore-avr GitHub Releases. The specific fixing commit can be reviewed at GitHub Pull Request #613. Additional details are available in the GitHub Security Advisory and Arduino Support Article ASEC-26-001.

Workarounds

• Implement application-level input validation to cap decimalPlaces values to reasonable limits (e.g., 6-10 digits)
• Avoid using external or untrusted input for floating-point precision parameters
• Consider using alternative string formatting methods with explicit buffer size limits
• Isolate critical systems from untrusted firmware sources until patching is complete

# Configuration example
# Update ArduinoCore-avr via Arduino IDE Board Manager
# 1. Open Arduino IDE
# 2. Navigate to Tools > Board > Boards Manager
# 3. Search for "Arduino AVR Boards"
# 4. Update to version 1.8.7 or later

# Alternative: Manual update via git
cd ~/Arduino/hardware/arduino/avr
git fetch origin
git checkout tags/1.8.7