CVE-2026-82404 — HIGH (8.3)

Vulnerability 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.

CVSS Score

8.3

HIGH

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

Attack Vector

NETWORK

Attack Complexity

LOW

Privileges Required

NONE

User Interaction

NONE

Scope

CHANGED

Confidentiality

LOW

Integrity

LOW

Availability

LOW

Related Weaknesses (CWE)

CWE-1321

References

FAQ

What is CVE-2026-82404?

CVE-2026-82404 is a vulnerability with a CVSS score of 8.3 (HIGH). 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...

How severe is CVE-2026-82404?

CVE-2026-82404 has been rated HIGH with a CVSS base score of 8.3/10. Review the CVSS metrics above for detailed severity breakdown.

Is there a patch for CVE-2026-82404?

Check the references section above for vendor advisories and patch information. Review vendor security bulletins for remediation guidance.