Vulnerability · Published Sep 30, 2026 · Assigned by GitHub_M

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

Critical 10.0

In plain English

A critical-severity code injection in kobako from elct9620. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.8% (higher than 57% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)10.0Critical

Exploit odds (30 days)0.8%top 43.3% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
RubyGems	kobako	0.9.1	GHSA-7pwq-q9jf-539h

Affected products (as reported)

Vendor	Product	Versions
elct9620	kobako	>= 0.1.0, < 0.9.1

Weakness: CWE-94, CWE-470 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

References

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