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, o…

CVE-2026-82404 is a prototype pollution vulnerability in the TOON serialization library, used for compact JSON encoding in LLM prompts. Prior to version 2.3.1, decoding attacker-controlled TOON data containing keys like __proto__, constructor, or prototype would write through the object prototype chain, polluting Object.prototype at runtime. The vulnerability affects multiple decode paths including expandPaths: 'safe', plain nested objects, tabular rows, quoted keys, and streaming decode. The encoder also had issues dropping own __proto__ properties and invoking inherited setters during normalization. Exploitation could lead to denial of service or, with a suitable downstream gadget, remote code execution. The fix was released in TOON version 2.3.1 with the patch available on GitHub.