CVE-2026-55107: Kobako sandbox escape via public_send reflection

CVE-2026-55107 · Severity: critical · CVSS 10 · Published 2026-09-30

Kobako is a Ruby gem that sandboxes untrusted mruby scripts by isolating them in a WebAssembly environment. A critical flaw allows guest scripts to execute arbitrary Ruby code in the host process, completely defeating the sandbox isolation. An attacker can read or modify any host data, spawn processes, and take full control of the host application.

Vendors: RubyGems.

Quick answers

Is CVE-2026-55107 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-55107 being exploited in the wild.
How severe is CVE-2026-55107?
Critical severity, with a CVSS score of 10, published 30 September 2026.
Which products and versions are affected?
kobako (RubyGems) >= 0.1.0, <= 0.9.0, ECOSYSTEM:introduced=0.1.0,fixed=0.9.1.
Is there a fix for CVE-2026-55107?
Yes. kobako (RubyGems): upgrade to 0.9.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-55107 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-55107?
Check whether you run kobako (RubyGems) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • kobako (RubyGems): upgrade to 0.9.1 or later
    bundle update kobako

Technical details

The vulnerability is a code injection flaw (CWE-94) combined with unsafe reflection (CWE-470). When guest scripts call methods on host-bound Service objects via the transport dispatcher, the dispatcher passes the guest-supplied method name directly to Ruby's `Object#public_send` without validation: `target.public_send(method.to_sym, *args, **kwargs, &block)`. This allows a guest to invoke any public method, including Ruby's ambient reflection methods. By requesting `method = "send"` with `args = [:eval, "<ruby>"]`, the guest achieves `target.send(:eval, "<ruby>")`, executing arbitrary host Ruby in the host process. No Service-specific behavior or authentication is required; any bound Service object enables the escape. The vulnerable code existed in versions 0.1.0 through 0.9.0 under three different names (`registry`, `rpc`, `transport`). Fixed in 0.9.1 by rejecting methods owned by core/meta modules (BasicObject, Kernel, Object, Module, Class), blocking all reflection entry points.

Affected products

  • elct9620 kobako 0.1.0 to 0.9.0

Identifiers

  • CVE-2026-55107
  • GHSA-7pwq-q9jf-539h

Timeline

  • 2026-08-18: disclosed
  • 2026-08-18: patched: Fixed in version 0.9.1

References

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