@stablelib/cbor: Prototype poisoning via `__proto__` map keys in CBOR decoding

Summary

### Summary `@stablelib/cbor` decodes CBOR maps into ordinary JavaScript objects and assigns attacker-controlled keys directly onto those objects. A CBOR map key named `__proto__` therefore changes the prototype of the decoded object instead of becoming an ordinary data property. ### Details The decoder builds map results with a plain `{}` and then stores attacker-controlled keys using bracket assignment. That is unsafe for special property names. In JavaScript, assigning to `obj["__proto__"]` on a normal object does not create a plain own property. It invokes the built-in `__proto__` setter and replaces the object’s prototype if the supplied value is an object or `null`. As a result, a CBOR payload containing a map entry like: * key: `"__proto__"` * value: `{ isAdmin: true }` does not decode to an object with an own property called `__proto__`. It decodes to an object whose prototype is now attacker-controlled. Any code that later reads properties through normal lookup will see inherited attacker-supplied values. ### PoC ```js import { decode } from "@stablelib/cbor"; // CBOR: // { // "__proto__": { "isAdmin": true } // } // // a1 map(1) // 69 text(9) // "__proto__" // a1 map(1) // 67 text(7) // "isAdmin" // f5 true const payload = new Uint8Array([ 0xa1, 0x69, 0x5f, 0x5f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x5f, 0x5f, 0xa1, 0x67, 0x69, 0x73, 0x41, 0x64, 0x6d, 0x69, 0x6e, 0xf5 ]); const obj = decode(payload); console.log(Object.hasOwn(obj, "isAdmin")); // false console.log(obj.isAdmin); // true console.log(Object.getPrototypeOf(obj).isAdmin); // true ``` ### Impact Any application that decodes untrusted CBOR into JavaScript objects can receive objects with attacker-controlled prototypes. In practice, that can corrupt configuration objects, influence authorization checks, alter feature flags, and break application logic that relies on normal property lookup instead of strict own-property checks. If the decoded object is later merged into other objects, the impact can spread further. ### Solution Upgrade to version 2.0.4.