Medium [CVE-2026-67319] Information disclosure and data manipulation via prototype pollution

axios before 0.33.0 (and 1.x before 1.18.0) can consume inherited properties from nested request option objects when the JavaScript process's Object.prototype has already been polluted by another component.

While the top-level merged config uses a null prototype, nested plain objects such as auth and paramsSerializer are cloned into ordinary objects and read without own-property checks.

When an application passes placeholder nested objects such as auth: {} or paramsSerializer: {}, inherited username/password values can cause silent injection of an Authorization: Basic header, and inherited encode/serialize values can alter query-string serialization (full serializer replacement requires a function-valued pollution primitive).

This is exploitable only in the presence of pre-existing prototype pollution. A flaw was found in axios.

When the JavaScript environment's `Object.prototype` has been previously compromised through a technique known as prototype pollution, axios can inadvertently use malicious properties from nested configuration objects. This can lead to the silent injection of unauthorized authentication headers, potentially exposing sensitive credentials.

Additionally, it could allow an attacker to alter how data is formatted for requests, leading to data manipulation.