CVE-2026-82404
Received Received - Intake

Prototype Pollution in TOON Serialization Library

Publication date: 2026-09-02

Last updated on: 2026-09-02

Assigner: GitHub, Inc.

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.

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Meta Information

Published
2026-09-02
Last Modified
2026-09-02
Generated
2026-09-02
AI Q&A
2026-09-02
EPSS Evaluated
N/A
NVD
CVE-2026-82404
EUVD
EUVD-2026-70214

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
toon-format toon 2.3.1

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-1321 The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.

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Executive Summary

CVE-2026-82404 is a prototype pollution vulnerability in the TOON library affecting versions before 2.3.1. It occurs when decoding untrusted TOON input containing keys like __proto__, constructor, or prototype. These keys can pollute the Object.prototype for the entire runtime, affecting all objects in the application.

Detection Guidance

To detect this vulnerability, check if your system uses the toon library version prior to 2.3.1. Run commands like 'npm list @toon-format/toon' or 'npm list toon' to verify the installed version. If the version is below 2.3.1, the system is vulnerable.

Impact Analysis

Services decoding untrusted TOON could experience denial of service or remote code execution if a downstream gadget is present. The vulnerability affects multiple input formats including nested objects, tabular rows, and quoted keys. The encoder may also drop own __proto__ properties or trigger inherited setters during normalization.

Compliance Impact

This vulnerability could potentially impact compliance with GDPR and HIPAA by enabling remote code execution or denial of service through prototype pollution. If exploited, it may lead to unauthorized data access, modification, or system disruption, violating confidentiality and integrity requirements under these regulations.

Mitigation Strategies

Immediately upgrade the toon library to version 2.3.1 or later using 'npm update @toon-format/toon' or 'npm update toon'. If upgrading is not possible, implement input validation to reject TOON data containing keys like __proto__, constructor, or prototype before decoding.

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