CVE-2026-82404

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

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TOON is a compact, human-readable serialization of JSON data for LLM prompts. Prior to 2.3.1, decoding attacker-controlled TOON with a __proto__, constructor, or prototype key wrote through the object prototype chain instead of creating an own property, polluting Object.prototype for the runtime. In packages/toon/src/decode/expand.ts, the expandPaths: 'safe' path and insertPathSafe function made dotted keys such as a.__proto__.x the strongest vector, while plain nested objects, tabular rows, quoted keys, and streaming decode were also affected. The encoder also dropped own __proto__ properties and could invoke an inherited setter during normalization. Services that decode untrusted TOON could experience denial of service or, when a suitable downstream gadget is present, remote code execution. This issue is fixed in version 2.3.1.

CVSS 8.3 · High · EPSS 0.7% · Fix available

Published

2026-09-02T16:30:54.876Z

Updated

2026-09-02T18:59:07.868Z

Stack: node

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