Vulnerabilities

Vulnerability · CVE-2026-55107

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

Published Aug 18, 2026 · catalog updated Oct 26, 2026 · kobako Sandbox Escape: guest

Last checked Oct 31, 2026. A connected source was read within the last 7 days.

Check if you run it

No confirmed exploitation is on record for CVE-2026-55107. EPSS is 0.80%. Act only if a listed version of kobako Sandbox Escape: guest is installed.

Severity

Critical10.0

Known exploitation

Not in CISA/VulnCheck KEV

EPSS

0.80%

Remediation

Advisory link

Affected products

RubyGems kobako

### 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 on the bound object, with no restriction to the object's own methods: can invoke any public method, including Ruby's ambient reflection surface. A guest pivots through the public into otherwise private Kernel methods: a dispatch request with and evaluates to , 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: executes in the host process and can read or modify host state, spawn processes, and so on. ### 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 sink under three successive names ( → → ). ### Patches Fixed in 0.9.1. The dispatcher now rejects any method whose resolved owner is a core/meta module ( , , , , ), so only methods the bound object itself defines — or dynamically handles via — remain reachable. The ambient reflection methods ( , , , , , , , …) are all owned by those modules and are blocked. ### 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.