Summary
kobako Sandbox Escape: guest eval reaches host RCE via methodmissing → publicsend (any bound Service)
A guest mruby script running inside the Kobako sandbox can execute arbitrary
Ruby in the host process, fully escaping the sandbox.
Details
A host embeds bound "Service" objects that guest scripts call across the wasm
boundary through the transport dispatcher. The dispatcher passed the
guest-supplied method name straight to Object#public_send on the bound
object, with no restriction to the object's own methods:
target.public_send(method.to_sym, *args, **kwargs, &block)
public_send can invoke any public method, including Ruby's ambient
reflection surface. A guest pivots through the public send into otherwise
private Kernel methods: a dispatch request with method = "send" andargs = [:eval, "<ruby>"] evaluates to target.send(:eval, "<ruby>"),
running attacker-controlled Ruby in the host. Any bound Service object is
sufficient, no Service-specific behavior is required.
Proof of Concept
A guest call equivalent to:
Service.send(:eval, "<arbitrary host ruby>")
executes in the host process and can read or modify host state, spawn
processes, and so on.
Workarounds
None within the affected versions. Until you can upgrade, do not bind any
host Service object into a sandbox that runs untrusted scripts. Upgrade to
0.9.1.
References
- GHSA-7pwq-q9jf-539h
- Fix commit: 64f8470
Credits
Reported and fixed by Ahmed Al Hafoudh.
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).
Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.
CVE-2026-55107 has a CVSS score of 10.0 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.9.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Fixed in 0.9.1. The dispatcher now rejects any method whose resolved owner is
a core/meta module (BasicObject, Kernel, Object, Module, Class), so
only methods the bound object itself defines, or dynamically handles viamethod_missing, remain reachable. The ambient reflection methods (send,__send__, public_send, instance_eval, instance_exec, method,instance_variable_get, …) are all owned by those modules and are blocked.
Frequently Asked Questions
- What is CVE-2026-55107? CVE-2026-55107 is a critical-severity code injection vulnerability in kobako (rubygems), affecting versions >= 0.1.0, <= 0.9.0. It is fixed in 0.9.1. Untrusted input is evaluated as executable code within the application's runtime environment.
- How severe is CVE-2026-55107? CVE-2026-55107 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of kobako are affected by CVE-2026-55107? kobako (rubygems) versions >= 0.1.0, <= 0.9.0 is affected.
- Is there a fix for CVE-2026-55107? Yes. CVE-2026-55107 is fixed in 0.9.1. Upgrade to this version or later.
- Is CVE-2026-55107 exploitable, and should I be worried? Whether CVE-2026-55107 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-55107 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-55107? Upgrade
kobakoto 0.9.1 or later.