# CVE-2026-86547: mrubyc through 4.0.0 NULL Pointer Dereference via OP_ENTER

> Attackers can crash applications embedding mrubyc by providing malicious bytecode, causing a denial of service via null pointer dereference.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 9 Sep 2026
- **Updated:** 9 Sep 2026
- **Also known as:** GHSA-42q4-rvcw-q7wc

## What is CVE-2026-86547?

CVE-2026-86547 is a medium-severity vulnerability in mrubyc, published on 9 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

mrubyc through 4.0.0 contains a null pointer dereference vulnerability in the op_enter() handler in src/vm.c when processing untrusted bytecode. Attackers can craft malicious .mrb bytecode files with OP_ENTER instructions at the top level to crash the embedding application and cause denial of service.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2026-86547?


## How do I fix CVE-2026-86547?

No fix has been published for CVE-2026-86547 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-86547 being exploited?

There is no known exploitation of CVE-2026-86547 in the wild and no public exploit code.

Public exploit code:

- Standalone proof of concept for CVE-2026-86547, a NULL pointer dereference in mrubyc op_enter() through 4.0.0. (github_poc): https://github.com/HarshRajSinghania/cve-2026-86547-mrubyc-op-enter

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-86547 and fix it if it is affected.

What it is: CVE-2026-86547 is a medium-severity vulnerability in mrubyc, published on 9 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-86547 in the wild and no public exploit code.

Affected product: mrubyc. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-86547/ (Markdown: https://insights.sbomify.com/CVE-2026-86547/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-86547
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-86547
- OSV: https://osv.dev/vulnerability/CVE-2026-86547

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-86547/
Summaries are generated and may contain mistakes; the official description above is the record.