EUVD-2026-77579
GHSA-325j-mg25-8q58
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
Remote, low-complexity, unauthenticated deserialization of untrusted documents; guaranteed integrity/availability impact via prototype pollution but no direct confidentiality loss, matching the advisory's 8.1.
Primary rating from Vendor (https://github.com/confetti/yayson).
CVSS VectorVendor: https://github.com/confetti/yayson
Lifecycle Timeline
5Blast Radius
ecosystem impact- 1 npm packages depend on yayson (1 direct, 0 indirect)
Ecosystem-wide dependent count for version 4.3.0.
DescriptionCVE.org
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 in3.0.0and4.2.0; 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] = modelThe 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 pollutedWorkarounds
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.
Articles & Coverage 1
AnalysisAI
Prototype pollution in the yayson npm JSON:API library (<= 4.2.0) lets a remote attacker corrupt Object.prototype by submitting a document whose resource type is "__proto__". Store/LegacyStore use the attacker-supplied type, id, and relationship names as plain-object keys in internal lookup tables, so a crafted JSON:API payload writes attacker-controlled key/value pairs onto every object in the Node.js process for its lifetime. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Requires that the target application deserializes attacker-influenced JSON:API documents through yayson's Store or LegacyStore (Store.sync or equivalent) at version <= 4.2.0. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The provided CVSS 3.1 vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H describes network-reachable, unauthenticated, low-complexity exploitation with no confidentiality impact but high integrity and availability impact - the advisory's own suggested severity is 8.1, while the summary score field lists 9.1; the 8.1 figure is the more defensible number given C:N. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Vendor-released patch: upgrade to yayson 4.3.0, which uses null-prototype lookup tables, rejects `__proto__`/`constructor`/`prototype` as document-derived member names, iterates with Object.keys(), and normalizes caller-supplied caches to null-prototype (per GHSA-325j-mg25-8q58). … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours, inventory all production dependencies on yayson (npm ls yayson) and immediately restrict external network access to identified services or take them offline pending remediation. …
Sign in for detailed remediation steps and compensating controls.
Threat intelligence, references, and detailed analysis are available after sign-in.
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External POC / Exploit Code
Leaving vuln.today