CVE-2026-82404: CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') in toon-format toon

TOON is a compact serialization format for JSON data used in LLM prompts. Versions of toon prior to 2.3.1 improperly handle keys such as __proto__, constructor, or prototype during decoding, writing through the object prototype chain instead of creating own properties. This prototype pollution occurs in the decode/expand.ts module, particularly via the expandPaths: 'safe' path and insertPathSafe function, which allow dotted keys like a.__proto__.x to modify Object.prototype. The encoder also mishandles own __proto__ properties, potentially invoking inherited setters. Exploiting this vulnerability can lead to denial of service or remote code execution if a downstream gadget is available. The issue is resolved in version 2.3.1.

Exploitation of this vulnerability can result in modification of the Object prototype, which may cause denial of service or remote code execution in environments decoding untrusted TOON data. The impact depends on the presence of suitable downstream gadgets to leverage the prototype pollution for code execution. Confidentiality, integrity, and availability are all potentially affected.

This vulnerability is fixed in toon version 2.3.1. Users should upgrade to version 2.3.1 or later to remediate the issue. No official patch links or advisories were provided, but upgrading to the fixed version is the recommended action.