{"id":"GHSA-7pwq-q9jf-539h","aliases":["CVE-2026-55107"],"title":"kobako Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)","description":"### Summary\nA guest mruby script running inside the Kobako sandbox can execute arbitrary\nRuby in the host process, fully escaping the sandbox.\n\n### Details\nA host embeds bound \"Service\" objects that guest scripts call across the wasm\nboundary through the transport dispatcher. The dispatcher passed the\nguest-supplied method name straight to `Object#public_send` on the bound\nobject, with no restriction to the object's own methods:\n\n```ruby\ntarget.public_send(method.to_sym, *args, **kwargs, &block)\n```\n\n`public_send` can invoke any public method, including Ruby's ambient\nreflection surface. A guest pivots through the public `send` into otherwise\nprivate Kernel methods: a dispatch request with `method = \"send\"` and\n`args = [:eval, \"<ruby>\"]` evaluates to `target.send(:eval, \"<ruby>\")`,\nrunning attacker-controlled Ruby in the host. Any bound Service object is\nsufficient — no Service-specific behavior is required.\n\n### Proof of Concept\nA guest call equivalent to:\n\n```\nService.send(:eval, \"<arbitrary host ruby>\")\n```\n\nexecutes in the host process and can read or modify host state, spawn\nprocesses, and so on.\n\n### Impact\nComplete sandbox escape leading to remote code execution in the host process,\ndefeating the gem's central guarantee of isolating untrusted mruby scripts.\nAny deployment that runs untrusted or attacker-influenced scripts is affected.\nAll released versions (0.1.0 through 0.9.0) are vulnerable; the dispatcher\ncarried the same unguarded `public_send` sink under three successive names\n(`registry` → `rpc` → `transport`).\n\n### Patches\nFixed in 0.9.1. The dispatcher now rejects any method whose resolved owner is\na core/meta module (`BasicObject`, `Kernel`, `Object`, `Module`, `Class`), so\nonly methods the bound object itself defines — or dynamically handles via\n`method_missing` — remain reachable. The ambient reflection methods (`send`,\n`__send__`, `public_send`, `instance_eval`, `instance_exec`, `method`,\n`instance_variable_get`, …) are all owned by those modules and are blocked.\n\n### Workarounds\nNone within the affected versions. Until you can upgrade, do not bind any\nhost Service object into a sandbox that runs untrusted scripts. Upgrade to\n0.9.1.\n\n### References\n- GHSA-7pwq-q9jf-539h\n- Fix commit: 64f8470\n\n### Credits\nReported and fixed by Ahmed Al Hafoudh.","severity":"critical","score":10,"published":"2026-08-18T20:09:59.000Z","modified":"2026-10-26T15:00:05.215Z","withdrawn":false,"knownExploited":false,"affected":[{"ecosystem":"RubyGems","name":"kobako","ranges":[{"events":[{"introduced":"0.1.0"},{"fixed":"0.9.1"}],"type":"ECOSYSTEM"}],"versions":["0.1.0","0.1.1","0.1.2","0.2.0","0.2.1","0.3.0","0.4.0","0.5.0","0.6.0","0.6.1","0.6.2","0.7.0","0.8.0","0.9.0"],"fixed":["0.9.1"]}],"references":["https://github.com/elct9620/kobako/security/advisories/GHSA-7pwq-q9jf-539h","https://nvd.nist.gov/vuln/detail/CVE-2026-55107","https://github.com/elct9620/kobako/commit/64f84700c81f44902bed9211318d5362f44987b3","https://github.com/elct9620/kobako","https://github.com/elct9620/kobako/releases/tag/v0.9.1","https://github.com/rubysec/ruby-advisory-db/blob/master/gems/kobako/GHSA-7pwq-q9jf-539h.yml"],"sources":[{"name":"OSV","id":"GHSA-7pwq-q9jf-539h","modified":"2026-10-26T15:00:05.215156345Z","url":"https://osv.dev/vulnerability/GHSA-7pwq-q9jf-539h"}],"assessments":[{"source":"OSV","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H","score":10}]}