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:

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

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.