CVE-2026-38973: Out-of-Bounds Read in mrubyc Method Lookup

Technical summary

The vulnerability is an out-of-bounds read in the mrbc_find_method() function that occurs during builtin missing-method lookup. When mrubyc attempts to resolve method calls that don't exist in the expected location, insufficient bounds checking allows the read operation to access memory outside the intended buffer. This is classified as a CWE-125 (Out-of-bounds Read) issue. The attack vector is local, requires no special privileges, and needs user interaction to trigger. The CVSS 3.1 score of 4.4 reflects low integrity impact potential—the vulnerability compromises confidentiality and availability without enabling code execution.

Business impact

Organizations using mrubyc in embedded systems or constrained environments face dual risks: information disclosure through memory leaks and denial of service via application crashes. For products embedding mrubyc in user-facing features, this could lead to unexpected failures or data exposure if the application processes sensitive values. The local-access requirement limits attack surface to scenarios where an attacker has shell or application-level access, but the user-interaction requirement means successful exploitation depends on user behavior rather than automatic triggering.

Affected systems

mrubyc releases through version 3.4.1 are affected. No explicit vendor or product list was provided in the advisory, suggesting this impacts direct consumers of the mrubyc project. Organizations should verify whether they ship mrubyc directly or depend on it as a transitive dependency in their software supply chain.

Exploitability

Exploitation requires local access and user interaction, making this a lower-risk vulnerability in practice. An attacker cannot trigger the flaw remotely or without privileged operation. The out-of-bounds read itself does not grant code execution; impact is limited to reading adjacent memory and potential application instability. No public exploit code or active exploitation has been documented at the time of this assessment.

Remediation

Upgrade mrubyc to a patched version released after 3.4.1. Consult the official mrubyc repository and release notes to identify the minimum version containing the bounds-checking fix. If immediate patching is not feasible, limit local access to the mrubyc runtime and isolate it from processing untrusted or user-supplied Ruby code.

Patch guidance

Check the mrubyc GitHub repository for releases published after July 9, 2026 (the modification date of this advisory) that address the mrbc_find_method() bounds issue. Apply the earliest patched release available to your environment. Verify the patch in release notes and commit logs before deployment. If you use mrubyc as a dependency, ensure your package manager or build tool fetches the updated version.

Detection guidance

Monitor application logs and system logs for crashes or unexpected exits in mrubyc processes, which may indicate out-of-bounds read attempts. If you have access to mrubyc source inspection, audit the mrbc_find_method() function for bounds-checking improvements in your version compared to the upstream repository. Runtime memory analysis tools may flag the out-of-bounds read if the vulnerability is triggered during testing, though detection in production is challenging without instrumentation.

Why prioritize this

This vulnerability merits moderate attention but not emergency patching. The CVSS 4.4 score and requirement for local access plus user interaction significantly limit real-world risk. However, any organization shipping mrubyc should plan a normal maintenance cycle upgrade within the next monthly patch window. Prioritize systems where mrubyc processes untrusted Ruby code or where memory exposure could affect confidential data.

Risk score, explained

The CVSS 3.1 score of 4.4 (MEDIUM) reflects the following factors: local attack vector eliminates remote exploitation; low privilege requirement; user interaction needed to trigger; confidentiality and availability impact but no integrity or code-execution capability. The score appropriately captures that this is a stability and information-leak issue rather than a critical system compromise vector.

Frequently asked questions