What’s Vulnerable › Vulnerabilities

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

• Severity: critical (CVSS 10.0 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-30 · updated 2026-09-30
• Weakness: CWE-94

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.

Affected products

• elct9620 kobako: from 0.1.0, < 0.9.1

References

• https://github.com/elct9620/kobako/commit/64f84700c81f44902bed9211318d5362f44987b3
• https://github.com/elct9620/kobako/releases/tag/v0.9.1
• https://github.com/elct9620/kobako/security/advisories/GHSA-7pwq-q9jf-539h
• https://github.com/elct9620/kobako/security/advisories/GHSA-7pwq-q9jf-539h

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