External risk intelligence

Kobako Sandbox Escape Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 10.0)

CVE-2026-55107

Kobako is a developer-focused Ruby library intended for embedding an isolated interpreter within applications. It is not an internet-facing service, appliance, or gateway itself; exposure depends entirely on how a developer implements it within their own application, making direct public-internet exposure of the library's internal sandbox mechanisms very unlikely in typical deployments.

Code Injection

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

The Kobako Ruby gem, designed to run untrusted code securely within applications, has a critical vulnerability that allows escaped code to execute arbitrary commands on the host system. This could lead to a complete compromise of the application's environment if the affected versions of Kobako are used to process external code inputs.

Code sandbox can be fully escaped.
Affects applications embedding untrusted code.
Confirm if your applications use this library.

Attack Path

How an attacker could exploit the issue

An attacker could compromise applications that use the Kobako Ruby gem by submitting specially crafted, untrusted scripts. If these scripts are executed within the Kobako sandbox, they could break out and run arbitrary Ruby code on the host system, potentially leading to a complete compromise of the application.

Requires an application using the gem.
Triggered by executing untrusted code.
Results in host process compromise.

Live Threat

Current exploitation, exposure, and threat context

A Ruby gem designed to run untrusted scripts in a sandbox can allow those scripts to fully escape the sandbox and execute arbitrary Ruby code within the host application when supported by the advisory's description. This could affect the integrity and availability of the host application and any data it processes.

Host application code and memory.
Untrusted scripts could execute arbitrary Ruby code.
Compromise of host application integrity and availability.

Operational Fix

Recommended remediation, mitigation, and detection steps

The Kobako Ruby gem, designed for embedding isolated mruby interpreters, has a critical sandbox escape vulnerability. This issue likely impacts development teams or platform teams responsible for integrating third-party code or custom scripts into applications. The immediate priority is to identify all applications utilizing Kobako, confirm their exposure and business criticality, and then coordinate remediation with the accountable application owners.

Application owners should own the issue.
Verify Kobako's presence and exposure.
Plan remediation based on risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the Kobako Ruby gem?

Kobako is a developer-focused tool used to embed a WebAssembly-isolated mruby interpreter within Ruby applications. It creates a secure sandbox that allows software to run untrusted code—such as user-provided formulas, LLM-generated scripts, or third-party plugins—without granting that code access to the host's sensitive resources like memory, files, or network connections.

How does CVE-2026-55107 break the sandbox?

This vulnerability involves improper control of code generation and execution, categorized as CWE-94 and CWE-470. In affected versions, the sandbox fails to fully isolate the guest script. Consequently, a script that should be confined can manipulate its environment to break out of the interpreter, allowing it to execute arbitrary Ruby code directly within the host process.

Do I need to run untrusted code for this to trigger?

Yes. This vulnerability is triggered specifically by the execution of a guest mruby script. If your application uses the Kobako gem but does not process or execute any untrusted inputs, plugins, or external scripts within that sandbox, the vulnerability cannot be triggered. The risk exists only when the sandboxed interpreter is actively tasked with running untrusted content.

How does Halo Surface Signal describe the risk?

Halo Surface Signal indicates that direct, public-internet exposure of the library's internal sandbox mechanisms is very unlikely. Because Kobako is an embedded library rather than an internet-facing service or gateway, the actual exposure depends on how a developer has implemented it inside their specific application.

Is there a standard fix for this vulnerability?

Yes. The vulnerability has been addressed by the maintainers in Kobako version 0.9.1. If your application relies on this gem, you should coordinate with your development or platform teams to identify where the library is being used, assess its business criticality, and update your dependencies to the patched version as soon as possible.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.