kobako Sandbox Escape - guest eval reaches host RCE via method_missing → public_send (any bound Service)
Published: August 18, 2026
SECURITY IDENTIFIERS
- CVE: CVE-2026-55107 (NVD)
- GHSA: GHSA-7pwq-q9jf-539h
- Vendor Advisory: https://github.com/elct9620/kobako/security/advisories/GHSA-7pwq-q9jf-539h
GEM
SEVERITY
CVSS v3.x: 10.0 (Critical)
PATCHED VERSIONS
>= 0.9.1
DESCRIPTION
Summary
A guest mruby script running inside the Kobako sandbox can execute arbitrary Ruby in the host process, fully escaping the sandbox.
Impact
Complete sandbox escape leading to remote code execution in the host process,
defeating the gem's central guarantee of isolating untrusted mruby scripts.
Any deployment that runs untrusted or attacker-influenced scripts is affected.
All released versions (0.1.0 through 0.9.0) are vulnerable; the dispatcher
carried the same unguarded public_send sink under three successive names
(registry → rpc → transport).
Credits
Reported and fixed by Ahmed Al Hafoudh.
RELATED
- https://rubygems.org/gems/kobako/versions/0.9.1
- https://github.com/elct9620/kobako/blob/main/CHANGELOG.md#091-2026-06-11
- https://github.com/elct9620/kobako/commit/dd081665f368f7ba54e476c3ad045ee1aa8ed703
- https://github.com/elct9620/kobako/commit/64f84700c81f44902bed9211318d5362f44987b3
- https://advisories.gitlab.com/gem/kobako/CVE-2026-55107
- https://github.com/elct9620/kobako/security/advisories/GHSA-7pwq-q9jf-539h
- https://github.com/advisories/GHSA-7pwq-q9jf-539h