GHSA-p95v-992w-h6c3

### Summary

Decoding attacker-controlled TOON containing a `__proto__`, `constructor`, or `prototype` key wrote through the object's prototype chain instead of creating an own property, polluting `Object.prototype` for the whole runtime. The `expandPaths: 'safe'` path (dotted keys such as `a.__proto__.x`) was the strongest vector; plain nested objects, tabular rows, and quoted keys were all affected. The encoder had a matching defect: it silently dropped own `__proto__` properties and could fire an inherited setter while normalizing.

### Impact

Any service that decodes untrusted TOON is affected. Prototype pollution can escalate to denial of service or, with a suitable downstream gadget, remote code execution.

### Patches

Upgrade to `@toon-format/toon@2.3.1`. Decoders now materialize `__proto__`/`constructor`/`prototype` as ordinary own data properties, matching `JSON.parse` semantics; the encoder preserves the same keys without invoking inherited accessors.

### Workarounds

None. Upgrade is the only fix. Callers who cannot upgrade should reject input whose keys include `__proto__`, `constructor`, or `prototype` before decoding.

### Port maintainers

The same bug shape can exist in any implementation that assigns decoded keys with `obj[key] = value`. Rust, Swift, Java, Python, and C# ports should audit their object-construction and path-expansion paths for the three prototype keys.