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

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:

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, "<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.

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).

Patches

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 via
method_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.

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.