+ "details": "### Summary\n\n`@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.\n\n### Details\n\nThe decoder builds map results with a plain `{}` and then stores attacker-controlled keys using bracket assignment.\n\nThat 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`.\n\nAs a result, a CBOR payload containing a map entry like:\n\n* key: `\"__proto__\"`\n* value: `{ isAdmin: true }`\n\ndoes 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.\n\n### PoC\n\n```js\nimport { decode } from \"@stablelib/cbor\";\n\n// CBOR:\n// {\n// \"__proto__\": { \"isAdmin\": true }\n// }\n//\n// a1 map(1)\n// 69 text(9)\n// \"__proto__\"\n// a1 map(1)\n// 67 text(7)\n// \"isAdmin\"\n// f5 true\n\nconst payload = new Uint8Array([\n 0xa1,\n 0x69, 0x5f, 0x5f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x5f, 0x5f,\n 0xa1,\n 0x67, 0x69, 0x73, 0x41, 0x64, 0x6d, 0x69, 0x6e,\n 0xf5\n]);\n\nconst obj = decode(payload);\n\nconsole.log(Object.hasOwn(obj, \"isAdmin\")); // false\nconsole.log(obj.isAdmin); // true\nconsole.log(Object.getPrototypeOf(obj).isAdmin); // true\n```\n\n### Impact\n\nAny application that decodes untrusted CBOR into JavaScript objects can receive objects with attacker-controlled prototypes.\n\nIn 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.",
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