All vulnerabilities
CVE-2026-55107GHSA-7pwq-q9jf-539h

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

elct9620 kobakoRubyGemsLibraries & packages
Open in ThreatlineRecord JSON
CVSS
10.0Critical
ExploitationPublic exploit codeCISA's assessment says proof of concept code is public, assessed 30 Sep 2026. Not in the KEV catalogCISA via CVE List
Exploit odds (EPSS)0.80%Chance of exploitation activity in the next 30 days, higher than 56% of CVEs. Scored 23 Oct 2026FIRST EPSS
Fixed in0.9.1Fixed versionOSV
Published30 Sep 2026Updated 30 Sep 2026CVE List

What fixes it

  • 0.9.1kobako, RubyGemsOSVbundle update kobako --conservative # needs 0.9.1 or later

A fixed version on one release line is not the upgrade target for every line. Check your exact version and any vendor backports.

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.

Text from CVE List

Affected versions 16

  • kobakoaffected: >= 0.1.0, < 0.9.1CVE List
  • kobako, RubyGemsfrom 0.1.0, fixed in 0.9.1OSV
  • kobako, RubyGems0.1.0OSV
  • kobako, RubyGems0.1.1OSV
  • kobako, RubyGems0.1.2OSV
  • kobako, RubyGems0.2.0OSV
  • kobako, RubyGems0.2.1OSV
  • kobako, RubyGems0.3.0OSV
  • kobako, RubyGems0.4.0OSV
  • kobako, RubyGems0.5.0OSV
  • kobako, RubyGems0.6.0OSV
  • kobako, RubyGems0.6.1OSV
  • kobako, RubyGems0.6.2OSV
  • kobako, RubyGems0.7.0OSV
  • kobako, RubyGems0.8.0OSV
  • kobako, RubyGems0.9.0OSV

References 3