CVE-2026-82404: TOON: Prototype pollution when decoding untrusted TOON input

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.