CVE-2026-61534 is a critical-severity (CVSS 9.1) CWE-1321 vulnerability in yayson. A fix is available for yayson — see the affected versions and patch details below.
Yayson: Prototype pollution in the Store/LegacyStore deserialization
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-61534.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-61534 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 398,142 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
yaysonnpmDescription
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] = 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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | yayson | all versions | 4.3.0npm install yayson@4.3.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for yayson, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update yayson to 4.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-61534 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Frequently Asked Questions
Is CVE-2026-61534 in your dependencies?
Find it across npm, including transitive dependencies.