TOON: Prototype pollution when decoding untrusted TOON input

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-82404
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-02 17:18:00 UTC
Updated2026-09-09 21:04:42 UTC
DescriptionTOON 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.

Risk And Classification

Primary CVSS: v3.1 8.3 HIGH from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L

EPSS: 0.003980000 probability, percentile 0.332470000 (date 2026-09-10)

Problem Types: CWE-1321 | CWE-1321 CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary8.3HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L
3.1CNADECLARED8.3HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Toon-format Toon affected < 2.3.1 Not specified

References

ReferenceSourceLinkTags
github.com/toon-format/toon/releases/tag/v2.3.1 [email protected] github.com
github.com/toon-format/toon/security/advisories/GHSA-p95v-992w-h6c3 [email protected] github.com
github.com/toon-format/toon/commit/94a2b7560b2b5ed903a4d466a3bce8b13daa2660 [email protected] github.com
github.com/toon-format/toon/pull/316 [email protected] github.com
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