CVE-2026-61534

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.

• CWE-1321

• https://github.com/confetti/yayson/commit/e460162d5a3ebac86424bead7424957c36f8dff9
• https://github.com/confetti/yayson/pull/111
• https://github.com/confetti/yayson/releases/tag/v4.3.0
• https://github.com/confetti/yayson/security/advisories/GHSA-325j-mg25-8q58
• https://github.com/confetti/yayson/security/advisories/GHSA-325j-mg25-8q58

Published

Sep 14, 2026

Last Modified

Sep 30, 2026

Status

Deferred

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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