The `mergeConfig()` function in Axios crashes with a TypeError when processing configuration objects that contain ` proto ` as an own property. This occurs because when ` proto ` is present (for example, when created via `JSON.parse()`), the function performs a prototype chain lookup that returns `Object.prototype` instead of a function, leading to a crash. A remote attacker can exploit this by providing a malicious configuration object, resulting in a complete denial of service. This issue affects Node.js servers and any backend that passes user-controlled JSON to Axios configuration methods.