Axios versions >=1.15.2 <1.18.0 have a prototype pollution read-side gadget in Basic auth subfield handling. When an axios request includes an auth object missing own username or password properties, axios reads these values from Object.prototype if polluted, causing attacker-controlled Basic auth headers to be sent. This occurs because nested auth subfields are accessed without own-property checks, allowing prototype chain reads. The vulnerability requires a separate prototype pollution primitive in the host application and specific axios usage patterns such as passing auth: opts.auth || {}. The impact is outbound request tampering, including injecting attacker-chosen Basic auth credentials or replacing existing Authorization headers. The vulnerability does not cause automatic credential exfiltration. The fix involves adding own-property checks when reading username and password fields, matching patterns used elsewhere in axios. Workarounds include avoiding empty or partial auth objects and filtering prototype pollution vectors in user input.

An attacker able to pollute Object.prototype.username and/or Object.prototype.password can influence the Authorization: Basic header on axios requests that pass an empty or partial auth object. This allows injection of attacker-chosen Basic auth credentials or replacement of existing Authorization headers, potentially causing downstream authorization failures or request tampering. The vulnerability does not directly disclose secrets from axios, and credential disclosure requires additional application-specific conditions. There is no evidence of known exploits in the wild. The severity is moderate due to the requirement of a separate prototype pollution vulnerability and specific axios usage patterns.

A fix is available for this vulnerability; users should upgrade axios to a version >=1.18.0 where the issue is patched. Until then, avoid passing empty or partial auth objects to axios requests. Applications should sanitize and filter untrusted input to prevent prototype pollution, especially filtering __proto__, constructor, and prototype properties. When merging user options, use own-property checks rather than fallback patterns like opts.auth || {}. Wrappers should use null-prototype objects or explicitly copy only own properties for auth objects. Check the official axios advisory or repository for the confirmed patched versions and upgrade guidance.