CVE-2026-61534: yayson Prototype Pollution Vulnerability

CRITICAL
Advisory Identifier: GHSA-325j-mg25-8q58
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CVSS 9.1
CWE-1321
Published
Advisory Summary

CVE-2026-61534 is a CRITICAL (CVSS 9.1) prototype pollution vulnerability (CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')) affecting yayson libraries in the npm ecosystem. The issue involves prototype pollution in store/legacystore deserialization. Remediation is available by upgrading affected packages to version 4.3.0.

CVE-2026-61534 at a Glance

CVE Identifier CVE-2026-61534
GHSA Identifier GHSA-325j-mg25-8q58
Severity CRITICAL
CVSS v3.1 Base Score 9.1
CWE Classification CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Affected Product yayson
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 9.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity High (I:H)
Availability High (A:H)

How to Fix CVE-2026-61534

To remediate this vulnerability, upgrade all affected yayson dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

yayson fixed: 4.3.0 
npm install yayson@4.3.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls yayson

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-61534, inspect your project dependencies using npm:

npm ls yayson

Analyze the terminal output to locate any version of yayson in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.3.0), your application is affected.

Note: The npm ls yayson command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'), where the product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. In yayson, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

# Summary `Store`/`LegacyStore` key internal lookup tables by the `type`, `id`, and relationship names from a JSON:API document. Because these were plain objects, a document with `type: "__proto__"` writes onto `Object.prototype`, polluting every object in the process. # Severity Suggested CVSS v3.1 `AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H` (8.1). The guaranteed impact is process-wide DoS / logic corruption; escalation to authorization bypass or RCE is dependent on gadgets in the consuming application. # Affected / Patched - Affected: `<= 4.2.0` (verified in `3.0.0` and `4.2.0`; all 3.x and 4.x). - Patched: `4.3.0`. # Details `type` is a string *value*, untouched by `JSON.parse`, and is used directly as an object key: ```js if (!models[type]) models[type] = {} // models["__proto__"] is Object.prototype → skipped if (!models[type][id]) models[type][id] = model // → Object.prototype[id] = model ``` The attacker controls the polluted key (`id`) and value (the model, populated from `attributes`). Pollution persists for the process lifetime. The malicious type can also arrive via an `included` resource referenced by a relationship, bypassing any `data.type` allow-list. `LegacyStore` is additionally reachable when a configured `types` mapping resolves to `"__proto__"`. # Proof of concept ```js const { Store } = require('yayson')() new Store().sync({ data: { type: '__proto__', id: 'polluted', attributes: { x: 1 } } }) console.log(({}).polluted) // { x: 1, id: 'polluted' } ← Object.prototype polluted ``` # Workarounds Reject documents whose `type` or relationship names are `__proto__`, `constructor`, or `prototype`, or run Node with `--disable-proto=throw`. # Fix Null-prototype lookup tables, rejection of `__proto__`/`constructor`/`prototype` as document-derived member names, `Object.keys()` iteration, and null-prototype normalization of caller-supplied caches.

CVE-2026-61534 Affected yayson Versions

1 component(s)
Fixed Versions:
4.3.0
Product / Package Ecosystem Affected Versions Fixed Version
yayson
npm/yayson
npm
< 4.3.0
4.3.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/confetti/yayson/security/advisories/GHSA-325j-mg25-8q58
yayson Source Repository REPOSITORY
https://github.com/confetti/yayson

Frequently Asked Questions

What is CVE-2026-61534?
CVE-2026-61534 is a security vulnerability in yayson involving Prototype pollution in Store/LegacyStore deserialization under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution').
Which yayson versions are affected?
Affected versions include releases prior to 4.3.0 across affected packages: yayson.
What version fixes CVE-2026-61534?
Upgrading to 4.3.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is yayson affected?
Yes, yayson is affected in releases prior to 4.3.0. Upgrade your dependency to 4.3.0 or later.
What is the CVSS score of CVE-2026-61534?
CVE-2026-61534 has a CVSS v3.1 base score of 9.1 (CRITICAL) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H.
What CWE is associated with CVE-2026-61534?
CVE-2026-61534 is classified under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') (The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.).
How can I check npm dependencies for CVE-2026-61534?
Run the dependency tree command: 'npm ls yayson'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-325j-mg25-8q58",
  "modified" : "2026-09-11T22:11:38Z",
  "published" : "2026-09-11T22:11:38Z",
  "aliases" : [ "CVE-2026-61534" ],
  "summary" : "yayson: Prototype pollution in Store/LegacyStore deserialization",
  "details" : "# Summary\n`Store`/`LegacyStore` key internal lookup tables by the `type`, `id`, and relationship names from a JSON:API document. Because these were plain objects, a document with `type: \"__proto__\"` writes onto `Object.prototype`, polluting every object in the process.\n\n# Severity\nSuggested CVSS v3.1 `AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H` (8.1). The guaranteed impact is process-wide DoS / logic corruption; escalation to authorization bypass or RCE is dependent on gadgets in the consuming application.\n\n# Affected / Patched\n- Affected: `<= 4.2.0` (verified in `3.0.0` and `4.2.0`; all 3.x and 4.x).\n- Patched: `4.3.0`.\n\n# Details\n`type` is a string *value*, untouched by `JSON.parse`, and is used directly as an object key:\n```js\nif (!models[type]) models[type] = {}          // models[\"__proto__\"] is Object.prototype → skipped\nif (!models[type][id]) models[type][id] = model // → Object.prototype[id] = model\n```\nThe attacker controls the polluted key (`id`) and value (the model, populated from `attributes`). Pollution persists for the process lifetime. The malicious type can also arrive via an `included` resource referenced by a relationship, bypassing any `data.type` allow-list. `LegacyStore` is additionally reachable when a configured `types` mapping resolves to `\"__proto__\"`.\n\n# Proof of concept\n```js\nconst { Store } = require('yayson')()\nnew Store().sync({ data: { type: '__proto__', id: 'polluted', attributes: { x: 1 } } })\nconsole.log(({}).polluted) // { x: 1, id: 'polluted' }  ← Object.prototype polluted\n```\n\n# Workarounds\nReject documents whose `type` or relationship names are `__proto__`, `constructor`, or `prototype`, or run Node with `--disable-proto=throw`.\n\n# Fix\nNull-prototype lookup tables, rejection of `__proto__`/`constructor`/`prototype` as document-derived member names, `Object.keys()` iteration, and null-prototype normalization of caller-supplied caches.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "yayson"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.3.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.2.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/confetti/yayson/security/advisories/GHSA-325j-mg25-8q58"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/confetti/yayson"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-11T22:11:38Z",
    "nvd_published_at" : null
  }
}