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

• Severity: high (CVSS 8.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-02 · updated 2026-09-09
• Weakness: CWE-1321

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: before 2.3.1

References

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

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