Preface: mruby/c remains active in certain industry scenarios that require Ruby’s rapid development capabilities but are limited by hardware constraints that standard mruby (which requires more memory) cannot support. It is widely used in:

• Internet of Things (IoT) edge devices.

• Factory automation and industrial sensors (especially favored by Japanese engineering companies).

• Custom firmware requiring lightweight, high-efficiency script engines embedded in C code.

Background: Which type of IoT devices use mruby/c?

mruby/c is used in resource-constrained, battery-powered IoT devices that require concurrent tasks but lack the memory to run a full operating system. Because its virtual machine runs on as little as 20 KB of RAM, it is deployed in hardware environments where other high-level languages like MicroPython or standard mruby (which requires >100 KB) are too heavy.

When a method lookup fails across the normal inheritance chain, the virtual machine invokes a specialized, static “builtin fallback routine” inside mrbc_find_method(). Because the bytecode execution pointer (pc) or target ID array indexes are processed directly within the loop logic without an explicit boundary assertion check, malicious or unexpectedly formatted bytecode can trigger an out-of-bounds read.

Why does a microcontroller use a Virtual Machine? Unlike a system virtual machine (like VMware or VirtualBox) that emulates an entire computer system or operating system, a programming language VM (like the Java Virtual Machine or the mruby/c VM) acts as a software execution engine.

•               The Process: Your Ruby code is first compiled into bytecode.

•               The VM’s Job: The mruby/c VM reads this bytecode step-by-step and translates it into physical instructions the ESP32 chip can understand.

Vulnerability details: mrubyc through release3.4.1 was found to contain an out-of-bounds read in builtin missing-method lookup inside mrbc_find_method().

Remedy: If you are compiling custom firmware using embedded mruby/c, you can fix this risk by following the repository changes:

1.             Pull the official fixed code directly from the mruby/c GitHub repository.

2.             Recompile your source directory to ensure that both the mrbc_find_method() out-of-bounds read and any related macro handlers are updated on your target MCU.

Official announcement: Please refer to the link for details – https://www.tenable.com/cve/CVE-2026-38973