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Critical, CVSS 10.0amended

kobako Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)

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

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (kobako 0.9.1), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • RubyGems kobako

    Affected
    >= 0.1.0, < 0.9.1
    Fixed in
    0.9.1

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
10.0 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
EPSS
  • CVE-2026-55107: 0.80% estimated exploitation activity in the next 30 days; percentile 56.59 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary 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: ```ruby 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"` and `args = [:eval, " "]` evaluates to `target.send(:eval, " ")`, 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, " ") ```…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 4 · amended ·

    Amended from osv record GHSA-7pwq-q9jf-539h

  2. Revision 3 · amended ·

    Amended from ghsa record GHSA-7pwq-q9jf-539h

  3. Revision 2 · amended ·

    Amended from ghsa record GHSA-7pwq-q9jf-539h

  4. Revision 1 · created ·

    Published from osv record GHSA-7pwq-q9jf-539h

Agent brief

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# Patch Next advisory brief: CVE-2026-55107

Advisory: kobako Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)
IDs: CVE-2026-55107, GHSA-7pwq-q9jf-539h
Record: https://patchnext.com/advisories/cve-2026-55107 (revision 4, source checked 27 Oct 2026, 06:25 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 10.0)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: kobako 0.9.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems kobako: affected >= 0.1.0, < 0.9.1; fixed in 0.9.1

## Suggested inspection (read-only)
- RubyGems kobako: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on kobako.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-7pwq-q9jf-539h (OSV record GHSA-7pwq-q9jf-539h lists affected ranges: RubyGems kobako >=0.1.0 <0.9.1 (fixed))
- Verified: https://github.com/advisories/GHSA-7pwq-q9jf-539h (GitHub-reviewed advisory GHSA-7pwq-q9jf-539h lists affected ranges: RubyGems kobako >=0.1.0 <=0.9.0)

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