CVE-2026-55107: kobako Sandbox Escape: guest eval reaches host RCE via method_missing → public_send (any bound Service)

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.8%, percentile 56: FIRST’s model estimates a 0.8% chance of exploitation activity in the next 30 days, higher than 56% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes kobako (RubyGems) >= 0.1.0, <= 0.9.0, fixed in 0.9.1. Check your lockfile, including transitive dependencies.

What it is

Kobako is a Ruby gem that embeds a Wasm-isolated mruby interpreter inside applications, allowing execution of untrusted Ruby scripts (LLM-generated code, user formulas, student submissions, third-party plugins) in-process without giving them access to host memory, files, network, or credentials. From version 0.1.0 to before version 0.9.1, a guest mruby script running inside the Kobako sandbox can execute arbitrary Ruby in the host process, fully escaping the sandbox. This issue has been patched in version 0.9.1.

From the GitHub advisory

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#publicsend on the bound object, with no restriction to the object's own methods:

publicsend 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:

executes in the host process and can read or modify host state, spawn processes, and so on.

Weakness type: CWE-94, CWE-470.

Product: kobako (RubyGems) · Assigned by: GitHub_M · Published: 2026-09-30

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.80% probability of exploitation activity in the next 30 days; percentile 56.

Timeline

• 2026-08-18 — GitHub-reviewed advisory GHSA-7pwq-q9jf-539h published (critical)
• 2026-09-30 — CVE record last updated
• 2026-09-30 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-7pwq-q9jf-539h github.com
2. CVE.org record CVE-2026-55107 cve.org
3. NVD entry CVE-2026-55107 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com

Critical RubyGems advisories · Today’s Security Digest