yayson CVE-2026-61534

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. The attacker must be able to set the document type to __proto__ in data or, notably, in an included resource referenced by a relationship - the latter bypasses allow-lists applied only to data.type. LegacyStore is additionally reachable when a configured types mapping resolves to __proto__. No authentication or user interaction is required by yayson itself (PR:N/UI:N). Limiting factors: the base impact is DoS/logic corruption only; escalation to authz bypass or RCE requires exploitable gadgets in the consuming application, and hardened runtimes (Node --disable-proto=throw) or input validation neutralize the primitive.

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. No CISA KEV listing and no EPSS score are provided, but a working proof-of-concept is published in the advisory, so exploit code exists even though there is no confirmation of exploitation in the wild. Real-world risk hinges on the consuming application: prototype pollution here reliably yields process-wide DoS or logic corruption, and any operator or deployment team running an internet-facing service that deserializes untrusted JSON:API through yayson should treat this as urgent, but escalation to authorization bypass or remote code execution is not guaranteed by yayson itself and requires exploitable gadgets in the surrounding code. The bar to trigger the base pollution is low, so priority should be driven by whether yayson processes attacker-supplied documents.

An attacker sends a crafted JSON:API document (for example a POST body or an included resource) with type: "__proto__" and a chosen id and attributes to an endpoint that deserializes input via yayson's Store. yayson writes the attacker's key/value onto Object.prototype, corrupting every object in the process and causing DoS or logic corruption, and potentially authorization bypass or RCE if a suitable gadget exists downstream. A working proof-of-concept is included in the published advisory.