CVE-2026-55107 Kobako Vulnerable to Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)
Attackers can escape the Kobako sandbox to execute arbitrary Ruby code on the host by exploiting method_missing and public_send flaws.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55107?
CVE-2026-55107 is a critical code injection vulnerability in elct9620 kobako, published on 30 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Which versions are affected by CVE-2026-55107?
2 open source packages ship an affected version, including kobako.
| Package | Affected | Fixed in |
|---|---|---|
kobako RubyGems | From 0.1.0 | 0.9.1 |
kobako RubyGems | From 0.1.0 | 0.9.1 |
How do I fix CVE-2026-55107?
Upgrade kobako to 0.9.1 or later.
Upgrade, and never evaluate templates, expressions or code built from user input. More about Code injection.
Is CVE-2026-55107 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55107 exists.
Sources
Also known as GHSA-7pwq-q9jf-539h.