Kobako Ruby gem: Sandbox escape enables host code execution
Summary
kobako is a Ruby gem that runs untrusted Ruby scripts in an isolated mruby interpreter within the host application. In versions from 0.1.0 before 0.9.1, a guest script can call methods through reflection that bypass the restriction to the bound Service object's own methods, which allows arbitrary Ruby code to run in the host process. An attacker who can get a script into the sandbox, for example through a formula feature, plugin system or AI tool, can therefore escape the sandbox without privileges or user interaction.
Affected
- kobako gem (RubyGems) from 0.1.0 before 0.9.1
Fixed in
- Version 0.9.1 (fix in commit 64f84700c81f44902bed9211318d5362f44987b3)
Workaround
- Until the gem is upgraded, do not bind host Service objects into sandboxes that run untrusted scripts.
- Optional network hardening: isolate affected applications in a restricted network segment with strict egress filtering. This limits the impact of an escape but does not fix the flaw.
Exploitation
- The vendor advisory is public and describes the mechanism; a proof-of-concept section is included.
- No evidence of exploitation in the wild was found; the CVE is not known to be in the CISA KEV catalog (not verified directly).
- CISA SSVC: exploitation status 'poc', automatable 'yes', technical impact 'total'.
Our recommendation
- Search Gemfile, Gemfile.lock and deployed images for kobako and check whether a version below 0.9.1 is present.
- Upgrade to 0.9.1 or later.
- Until patched, stop binding host Service objects into sandboxes that run untrusted code.
- Review features that accept user-supplied or AI-generated Ruby code and restrict them if possible.
- Monitor Ruby services using kobako for unexpected child processes or outbound connections.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- CVE-2026-55107 fix commit github.com
- kobako release v0.9.1 github.com
- GHSA-7pwq-q9jf-539h vendor advisory github.com
- GitLab advisory CVE-2026-55107 advisories.gitlab.com
- RubySec advisory CVE-2026-55107 rubysec.com
- OSV CVE-2026-55107 osv.dev
- RubyGems kobako rubygems.org
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 30 Oct 2026, 14:44.
Original description (NVD)
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.
NVD references (4)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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