---
title: "CVE-2026-55107: kobako Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)"
url: https://erroneo.us/security/cve/CVE-2026-55107/
description: "CVE-2026-55107 (kobako (RubyGems)): Kobako is a Ruby gem that embeds a Wasm-isolated mruby interpreter inside applications, allowing execution of untrusted…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-09-30
date_modified: 2026-09-30
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-55107"
kev: false
cvss: 10
epss: 0.00799
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-55107: kobako Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.8% , percentile 56: FIRST’s model estimates a 0.8% chance of exploitation activity in the next 30 days, higher than 56% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes kobako (RubyGems) >= 0.1.0, <= 0.9.0, fixed in 0.9.1 . Check your lockfile, including transitive dependencies.

## What it is

Kobako is a Ruby gem that embeds a Wasm-isolated mruby interpreter inside applications, allowing execution of untrusted Ruby scripts (LLM-generated code, user formulas, student submissions, third-party plugins) in-process without giving them access to host memory, files, network, or credentials. From version 0.1.0 to before version 0.9.1, a guest mruby script running inside the Kobako sandbox can execute arbitrary Ruby in the host process, fully escaping the sandbox. This issue has been patched in version 0.9.1.

## Affected packages

- RubyGems `kobako` >= 0.1.0, <= 0.9.0 → fixed: 0.9.1

## Affected products (CVE record)

- elct9620 kobako: >= 0.1.0, < 0.9.1

## Severity

- GitHub_M CVSS 3.1: 10 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H`
- GitHub CVSS 3.1: 10.0 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.80% probability of exploitation activity in the next 30 days; percentile 56.

## Sources

- [GitHub Advisory GHSA-7pwq-q9jf-539h](https://github.com/advisories/GHSA-7pwq-q9jf-539h)
- [CVE.org record CVE-2026-55107](https://www.cve.org/CVERecord?id=CVE-2026-55107)
- [NVD entry CVE-2026-55107](https://nvd.nist.gov/vuln/detail/CVE-2026-55107)
- [github.com](https://github.com/elct9620/kobako/security/advisories/GHSA-7pwq-q9jf-539h)
- [github.com](https://github.com/elct9620/kobako/commit/64f84700c81f44902bed9211318d5362f44987b3)
- [github.com](https://github.com/elct9620/kobako/releases/tag/v0.9.1)

---
Source page: https://erroneo.us/security/cve/CVE-2026-55107/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt