Kobako Vulnerable to Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service) (CVE-2026-55107) | HOL Guard CVE

Critical · CVSS 10.0CVE-2026-55107GHSA-7PWQ-Q9JF-539H

Kobako Vulnerable to Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)

Answer in brief

CVE-2026-55107 records a Critical severity (CVSS 10.0) vulnerability in Kobako Vulnerable to Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service). The current sources do not mark it as known exploited. The current feed maps elct9620/kobako (generic), kobako (rubygems). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 30, 2026Updated Sep 30, 2026Source checked Sep 30, 2026First seen by HOL Aug 18, 2026Material review Sep 30, 2026

Upstream Advisory

Key facts

Risk: Critical · CVSS 10.0
Exploitation: Not marked as known exploited
Affected software: 2 mapped packages or products
Fix availability: Available

Why this deserves its current priority

CVSS is 10.0. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps elct9620/kobako (generic), kobako (rubygems). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
elct9620/kobakogeneric	>=0.1.0 <0.9.1	0.9.1
kobakorubygems	>=0.1.0,<=0.9.0	0.9.1

Recommended response

Check inventory. Check lockfiles and deployed manifests for elct9620/kobako, kobako.
Review the reported fix. Update elct9620/kobako to 0.9.1; kobako to 0.9.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Sep 30, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 30, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Aug 18, 2026

Sources and claim methodology

Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.