Unintended Proxy or Intermediary ('Confused Deputy')

Summary

The Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into a full Man-in-the-Middle (MITM) attack -- intercepting, reading, and modifying all HTTP traffic including authentication credentials. The HTTP adapter at lib/adapters/http.js:670 reads config.proxy via standard property access, which traverses the prototype chain. Because proxy is not present in Axios defaults, the merged config object has no own proxy property, making it trivially injectable via prototype pollution. Once injected, setProxy() routes all HTTP requests through the attacker's proxy server. Unlike the transformResponse gadget (which is constrained by assertOptions to return true), the proxy gadget has zero constraints -- the attacker gets a full MITM position with the ability to read all credentials and tamper with all responses. All versions from 1.0.0 prior to 1.16.0 are affected.