CVE-2026-82404
HighCVSS 8.3Exploitation Probability (EPSS)
Low risk33th percentile - higher than 33% of all known CVEs
Summary
TOON, a serialization of JSON for LLM prompts, prior to 2.3.1 has a prototype pollution vulnerability. Decoding attacker-controlled TOON with __proto__, constructor, or prototype keys writes through the object prototype chain instead of creating an own property, polluting Object.prototype. This could lead to denial of service or remote code execution if a suitable gadget is present.
Risk Assessment
Services that decode untrusted TOON data may experience denial of service or remote code execution, potentially compromising system security and availability.
Recommendation
It is recommended to upgrade to version 2.3.1 or later, which contains the fix eliminating the vulnerability.
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Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

