# CVE-2026-82404

## Summary

- **CVE ID:** CVE-2026-82404
- **Severity:** HIGH
- **CVSS Score:** 8.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L)
- **CWE:** CWE-1321
- **Published:** Sep 2, 2026
- **Last Modified:** Sep 2, 2026

## Description

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.

## Affected Products

- toon-format — toon (< 2.3.1)

## References

- [CNA](https://github.com/toon-format/toon/security/advisories/GHSA-p95v-992w-h6c3)
- [CNA](https://github.com/toon-format/toon/pull/316)
- [CNA](https://github.com/toon-format/toon/commit/94a2b7560b2b5ed903a4d466a3bce8b13daa2660)
- [CNA](https://github.com/toon-format/toon/releases/tag/v2.3.1)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.40%
- **EPSS Percentile:** 33.2

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._