Axios versions >=1.15.2 <1.18.0 have a vulnerability where Basic auth subfields (username and password) are read without own-property checks, allowing prototype pollution in Object.prototype.username and Object.prototype.password to influence the Authorization header in outbound requests. This occurs when axios receives an auth object lacking these own properties, causing it to read inherited polluted values. The vulnerability affects Node HTTP adapter and browser/fetch helpers. Exploitation requires a pre-existing prototype pollution vulnerability in the host environment and axios calls passing empty or partial auth objects. The fix involves adding own-property checks to these subfield reads.

An attacker able to pollute Object.prototype.username or Object.prototype.password can cause axios to send attacker-chosen Basic auth credentials in outbound requests when the auth object is empty or missing these fields. This can tamper with outbound requests by injecting or replacing Authorization headers, potentially causing authorization failures or unintended access. However, this does not automatically disclose legitimate credentials from axios or the application. Credential disclosure would require additional application-specific conditions such as observable request destinations and partial real auth objects.

No official patch or fix is currently confirmed. To mitigate, avoid passing empty or partial auth objects to axios requests. Only set auth when both username and password are explicitly provided as own properties. Applications merging untrusted input should filter prototype pollution vectors such as __proto__, constructor, and prototype. Use null-prototype objects or explicitly copy own properties when materializing optional auth objects. Monitor vendor advisories for official fixes and apply them when available.