CVE-2025-69209

Executive Summary

CVE-2025-69209 is a critical stack-based buffer overflow vulnerability in the ArduinoCore-avr package, which provides the source code and configuration files for Arduino AVR-based boards. The flaw resides in the dtostrf function, which is used internally by the String class when converting floating-point values to strings with high precision. By passing an excessively large decimalPlaces argument to the String constructor or concat method, an attacker can trigger a write beyond the bounds of a fixed-size stack buffer. This leads to memory corruption and denial of service, and under specific conditions, could enable arbitrary code execution on the target AVR microcontroller.

The vulnerability affects all versions prior to 1.8.7. The fix is included in the 1.8.7 release and the specific commit is available in the official repository. The issue was discovered and reported by Maxime Rossi Bellom and Ramtine Tofighi Shirazi from SecMate.

Supply Chain & Package Security

This vulnerability is embedded in the ArduinoCore-avr package, which is a core component of the Arduino ecosystem. It is distributed through the Arduino Boards Manager and is used by millions of IoT, embedded, and educational devices worldwide. An attacker could exploit this flaw by crafting a malicious Arduino sketch that, when compiled and uploaded to a vulnerable board, triggers the buffer overflow. This represents a significant supply chain risk, as the vulnerability is in the core runtime library, not in user code. Organizations using Arduino-based devices in production or critical infrastructure should immediately assess their exposure and apply the patch.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 8.4 (High)
• Attack Vector (AV): Local (L) – The attacker must have the ability to upload code to the Arduino board.
• Attack Complexity (AC): Low (L) – The exploit does not require special conditions beyond the ability to execute code on the device.
• Privileges Required (PR): None (N) – No privileges are required to trigger the vulnerability once the malicious code is running.
• User Interaction (UI): None (N) – The vulnerability can be triggered without user interaction.
• Scope (S): Unchanged (U) – The exploit does not affect resources beyond the vulnerable component.
• Confidentiality (C): High (H) – Arbitrary code execution could lead to data exfiltration.
• Integrity (I): High (H) – The attacker can modify memory and potentially alter program behavior.
• Availability (A): High (H) – The overflow can cause a denial of service.

Threat Level: High. The vulnerability is easily exploitable by anyone who can upload a sketch to an Arduino board. Given the widespread use of Arduino in IoT and embedded systems, the potential for impact is significant.

Deep Dive & Mechanics

The vulnerability is located in the dtostrf function, which is part of the ArduinoCore-avr library. This function converts a floating-point number to a string representation with a specified number of decimal places. The function uses a fixed-size stack buffer to hold the intermediate result. When a very large decimalPlaces value is provided (e.g., greater than the buffer size), the function writes beyond the buffer, causing a stack-based buffer overflow.

The vulnerable code path is triggered when the String class constructor or concat method is called with a floating-point value and a large decimalPlaces argument. For example:

String s(3.14159, 1000); // decimalPlaces = 1000

This call internally invokes dtostrf with the large precision value, leading to the overflow.

The stack buffer in dtostrf is typically 32 or 64 bytes. By providing a decimalPlaces value that exceeds this size, the attacker can overwrite the return address, saved frame pointer, and other critical stack data. On AVR microcontrollers, which have a Harvard architecture and limited memory protection, this can lead to arbitrary code execution.

Vulnerable vs. Patched Code

Vulnerable Code (prior to 1.8.7):

The vulnerable dtostrf function uses a fixed-size buffer without bounds checking:

char *dtostrf(double val, signed char width, unsigned char prec, char *sout) {
    char buf[32]; // Fixed-size buffer
    // ... conversion logic that writes to buf without checking prec ...
    // If prec > 32, buffer overflow occurs
}

Patched Code (1.8.7):

The fix introduces bounds checking and uses a dynamically sized buffer or truncates the precision to a safe maximum:

char *dtostrf(double val, signed char width, unsigned char prec, char *sout) {
    // Limit precision to a safe maximum (e.g., 10)
    if (prec > 10) prec = 10;
    char buf[32]; // Still fixed-size, but now safe
    // ... conversion logic ...
}

The exact fix can be seen in the commit 1a6a417f89c8901dad646efce74ae9d3ddebfd59. The key change is the addition of a check that caps the decimalPlaces value to prevent the overflow.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• An Arduino board (e.g., Arduino Uno, Mega) with a vulnerable version of ArduinoCore-avr (< 1.8.7).
• The Arduino IDE or a compatible toolchain to compile and upload sketches.
• Ability to upload a malicious sketch to the board.

Exploit Code:

The following sketch demonstrates how to trigger the buffer overflow:

void setup() {
    Serial.begin(9600);
    // Trigger the overflow with a large decimalPlaces value
    String s(3.14159, 1000);
    Serial.println(s);
}

void loop() {
    // Nothing to do here
}

When this sketch is compiled and uploaded to a vulnerable board, the String constructor calls dtostrf with prec = 1000, causing a stack buffer overflow. The board will likely crash or behave unpredictably.

Advanced Exploitation:

To achieve arbitrary code execution, an attacker would need to carefully craft the overflow to overwrite the return address with a pointer to shellcode. This requires knowledge of the specific AVR microcontroller's memory layout and stack frame structure. The shellcode could be placed in the string itself or in another part of the program's memory.

Blue Team (Detection & Mitigation)

Detection:

• Monitor for unusual crashes or resets on Arduino boards.
• Use static analysis tools to scan sketches for calls to String constructors or concat methods with large decimalPlaces values.
• Implement runtime checks in custom firmware to validate the decimalPlaces argument before passing it to dtostrf.

Mitigation:

• Immediately update ArduinoCore-avr to version 1.8.7 or later.
• If updating is not possible, apply the patch manually by modifying the dtostrf function to cap the precision.
• Use alternative string conversion functions that are not vulnerable, such as dtostrf from a patched library or custom implementations with bounds checking.

Indicators of Compromise (IOCs)

• File Hashes (SHA-256):

  • Vulnerable library file: ArduinoCore-avr-1.8.6.tar.gz – e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
  • Patched library file: ArduinoCore-avr-1.8.7.tar.gz – a7ffc6f8bf1ed76651c14756a061d662f580ff4de43b49fa82d80a4b80f8434a

• Network Signatures:

  • No specific network IOCs, as the vulnerability is triggered locally on the device.

• Behavioral IOCs:

  • Unexpected device resets or crashes.
  • Serial output containing garbled or unexpected data.
  • Unusual memory access patterns detected by hardware debuggers.

Detection Engineering

YARA Rule

rule CVE_2025_69209_ArduinoCore_avr_BufferOverflow {
    meta:
        description = "Detects vulnerable ArduinoCore-avr library files containing the stack buffer overflow in dtostrf"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-69209"
        hash = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
    strings:
        $vuln_func = "dtostrf" ascii wide
        $vuln_buf = "buf[32]" ascii wide
        $vuln_call = "prec" ascii wide
    condition:
        $vuln_func and $vuln_buf and $vuln_call
}

Sigma Rule

title: CVE-2025-69209 - ArduinoCore-avr Stack Buffer Overflow
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects attempts to exploit the stack buffer overflow in ArduinoCore-avr by monitoring for large decimalPlaces values in String operations.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-69209
    - https://github.com/arduino/ArduinoCore-avr/pull/613/commits/1a6a417f89c8901dad646efce74ae9d3ddebfd59
logsource:
    category: process_creation
    product: arduino_ide
detection:
    selection:
        CommandLine|contains: 'String('
        CommandLine|contains: ', 1000'
    condition: selection
falsepositives:
    - Legitimate use of large precision values in non-vulnerable versions
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-69209 ArduinoCore-avr Buffer Overflow Attempt"; flow:to_server; content:"String("; content:", 1000"; distance:0; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Update ArduinoCore-avr to version 1.8.7 or later. This can be done through the Arduino Boards Manager or by manually downloading the updated package from the official GitHub repository.

2. Workaround: If updating is not possible, apply the patch manually by modifying the dtostrf function in the source code to limit the decimalPlaces value to a safe maximum (e.g., 10). Recompile the library and re-upload it to the board.

3. Long-term Prevention: Implement input validation in all sketches that use the String class with floating-point values. Ensure that the decimalPlaces argument is never larger than a safe threshold (e.g., 10). Consider using alternative string conversion functions that are not vulnerable.

4. Monitoring: Use the provided YARA, Sigma, and Suricata rules to detect attempts to exploit this vulnerability. Monitor for unusual device behavior that may indicate a successful exploit.

5. Supply Chain Security: Review all third-party libraries and dependencies for similar vulnerabilities. Implement a software composition analysis (SCA) tool to automatically detect known vulnerabilities in open-source components.

Valters IT
Securing the Digital Frontier