---
title: "CVE-2026-61534: yayson: Prototype pollution in Store/LegacyStore deserialization"
url: https://erroneo.us/security/cve/CVE-2026-61534/
description: "CVE-2026-61534 (yayson (npm)): Yayson is a library for serializing and reading JSON API data in JavaScript. Prior to 4.3.0, Store and LegacyStore use…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-09-14
date_modified: 2026-09-14
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-61534"
kev: false
cvss: 9.1
epss: 0.0084
license: All rights reserved. Quote with attribution and a link.
---

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

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.8% , percentile 57: FIRST’s model estimates a 0.8% chance of exploitation activity in the next 30 days, higher than 57% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes yayson (npm) <= 4.2.0, fixed in 4.3.0 . Check your lockfile, including transitive dependencies.

## What it is

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.

## Affected packages

- npm `yayson` <= 4.2.0 → fixed: 4.3.0

## Affected products (CVE record)

- confetti yayson: < 4.3.0

## Severity

- GitHub_M CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H`
- GitHub CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact partial .
- FIRST EPSS (model of 2026-10-21): 0.84% probability of exploitation activity in the next 30 days; percentile 57.

## Sources

- [GitHub Advisory GHSA-325j-mg25-8q58](https://github.com/advisories/GHSA-325j-mg25-8q58)
- [CVE.org record CVE-2026-61534](https://www.cve.org/CVERecord?id=CVE-2026-61534)
- [NVD entry CVE-2026-61534](https://nvd.nist.gov/vuln/detail/CVE-2026-61534)
- [github.com](https://github.com/confetti/yayson/security/advisories/GHSA-325j-mg25-8q58)
- [github.com](https://github.com/confetti/yayson/pull/111)
- [github.com](https://github.com/confetti/yayson/commit/e460162d5a3ebac86424bead7424957c36f8dff9)
- [github.com](https://github.com/confetti/yayson/releases/tag/v4.3.0)

---
Source page: https://erroneo.us/security/cve/CVE-2026-61534/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt