The vulnerability stems from the $.deparam function's handling of nested parameter keys. The code splits keys using bracket notation (e.g., 'a[b][c]') and recursively builds object structures. When processing keys like 'proto[polluted]' or 'constructor[prototype][polluted]', it improperly assigns values to Object.prototype due to lack of prototype chain validation. The provided PoC and code analysis from BlackFan's repository confirm this vector. The function's role in parsing untrusted URL parameters makes it the clear entry point for prototype pollution.