toon-format toon up to 2.3.0 Decode expand.ts insertPathSafe prototype pollution

Summaryinfo

A vulnerability has been found in toon-format toon up to 2.3.0 and classified as critical. This vulnerability affects the function insertPathSafe of the file packages/toon/src/decode/expand.ts of the component Decode. Performing a manipulation results in prototype pollution. This vulnerability is reported as CVE-2026-82404. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability was found in toon-format toon up to 2.3.0. It has been classified as critical. This affects the function insertPathSafe of the file packages/toon/src/decode/expand.ts of the component Decode. The manipulation with an unknown input leads to a prototype pollution vulnerability. CWE is classifying the issue as 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. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-82404 since 08/29/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1059 according to MITRE ATT&CK.

Upgrading to version 2.3.1 eliminates this vulnerability.

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Productinfo

Vendor

• toon-format

Name

• toon

Version

• 2.0
• 2.1
• 2.2
• 2.3.0

Website

• Product: https://github.com/toon-format/toon/

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