CVE-2026-44490: axios has DoS & Header Injection via Prototype Pollution Read-Side Gadgets in axios merge functions

axios 1.15.2 exposes two read-side prototype-pollution gadgets. When Object.prototype is polluted by an upstream dependency in the same process (e.g. lodash _.merge / CVE-2018-16487), axios silently picks up the polluted values:

1. Header injection - lib/utils.js line 406 builds merge()’s accumulator as result = {}, so result[targetKey] (line 414) walks Object.prototype and the polluted bucket’s own keys are copied into the merged headers and ride out on the wire.
2. Crash DoS - lib/core/mergeConfig.js line 26 builds the hasOwnProperty descriptor as a plain-object literal. Object.defineProperty reads descriptor.get/descriptor.set via the prototype chain, so a polluted Object.prototype.get or Object.prototype.set makes the call throw TypeError synchronously on every axios request.