Kobako Vulnerable to Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- Code execution
- Confidentiality · integrity · availability
- High · High · High
- Fix status
- Fixed
Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub. Impact type from CWE-94.
Summary
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- Poc
- Automatable
- Yes
- Technical impact
- Total
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| elct9620 | kobako | 0.1.0 to before 0.9.1 | 0.9.1 | Patch available | CNA |
| — | kobako (RubyGems) | 0.1.0 to before 0.9.1 | 0.9.1 | Patch available | GHSA-7pwq-q9jf-539h |
- kobako Vendorelct9620 Affected 0.1.0 to before 0.9.1 Fixed in 0.9.1 Patch available CNA
- kobako (RubyGems) Vendor— Affected 0.1.0 to before 0.9.1 Fixed in 0.9.1 Patch available GHSA-7pwq-q9jf-539h
Remediation
- Fix · CNA kobako (elct9620) Fixed in 0.9.1
- Fix · GHSA-7pwq-q9jf-539h kobako (RubyGems) Fixed in 0.9.1 Advisory →
Timeline
- 2026-10-02 Threatist record created from CVE-2026-55107
- 2026-10-03 Alias added: GHSA-7pwq-q9jf-539h
References
Other 5
- https://github.com/elct9620/kobako/security/advisories/GHSA-7pwq-q9jf-539h
- https://github.com/elct9620/kobako/commit/64f84700c81f44902bed9211318d5362f44987b3
- https://github.com/elct9620/kobako/releases/tag/v0.9.1
- https://github.com/elct9620/kobako
- https://github.com/rubysec/ruby-advisory-db/blob/master/gems/kobako/GHSA-7pwq-q9jf-539h.yml
Sources
- CVE-2026-55107 · CVE Program
- CVE-2026-55107 · NVD
- FIRST EPSS
- GHSA-7pwq-q9jf-539h · GitHub Advisory Database · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences