This operation writes directly onto the Object.prototype, affecting every object within the Node.js process lifetime. The vulnerability is highly impactful as it enables logic corruption or denial of service by design. Furthermore, if the host application contains suitable gadget chains, this pollution can be escalated to arbitrary code execution or authorization bypass. The vulnerability persists even when using included resources or custom type mappings, making it difficult to mitigate through standard input validation if the application relies on deeply nested data structures.

Attack Chain

1. Attacker crafts a malicious JSON:API document containing a data object with type set to __proto__.
2. The target application receives the document via a network request and passes it to the yayson Store.sync() or LegacyStore deserialization method.
3. yayson parses the JSON:API object and processes the type string as a property key for the internal models lookup table.
4. The library performs an assignment operation: models["__proto__"][id] = model, which effectively injects properties into the global Object.prototype because __proto__ references the prototype of the models object.
5. The attacker includes malicious payloads within the attributes or id fields of the JSON:API document, which are then persisted globally across the process.
6. The application performs subsequent object operations that trigger the injected malicious properties, leading to code logic alterations or unauthorized state changes.
7. The attacker leverages existing gadget chains in the application environment to execute arbitrary code or bypass security controls based on the polluted prototype properties.

Impact

The vulnerability allows an unauthenticated attacker to permanently alter the behavior of the Node.js application process. Successful exploitation leads to process-wide logic corruption and denial of service. Depending on the downstream application code, the impact can extend to authorization bypass or full remote code execution. This affects all users of yayson versions 3.x and 4.x up to and including 4.2.0.

Recommendation

Prioritized actions for engineering teams:

• Update the yayson library to version 4.3.0 or later to include null-prototype lookup table implementations.
• Implement a temporary mitigation by rejecting incoming JSON documents where type or relationship names are set to __proto__, constructor, or prototype.
• Execute Node.js processes with the --disable-proto=throw flag to prevent prototype access if the environment permits.
• Audit applications using yayson to identify usage of deep object merging or deserialization of untrusted user-supplied JSON:API payloads.