## Summary

Axios fetch adapter requests can be altered by inherited properties on `fetchOptions`. The adapter resolves method, headers, body, signal, and credentials into `resolvedOptions`, creates a `Request`, and then calls `fetch(request, fetchOptions)` instead of `fetch(request, resolvedOptions)`. In runtimes such as Node's undici-backed fetch, inherited `fetchOptions.headers` can override the headers already placed on the `Request`.

Axios does not create the prototype pollution source. This is a read-side gadget that becomes exploitable after same-process prototype pollution.

## Impact

An attacker with a prior prototype-pollution primitive can cause affected fetch-adapter requests to send attacker-controlled headers and drop caller-specified headers. This can affect authorization, cache behavior, metadata-service interactions, or application-specific header-based controls.

The issue is specific to fetch-adapter behavior and does not affect Node HTTP adapter requests.

## Affected Functionality

Affected:

- `adapter: 'fetch'`.
- Runtime environments where the fetch adapter is selected.
- Requests where `fetchOptions` is an object that does not have safe own values for sensitive fetch init fields.

Not affected:

- Node HTTP adapter.
- Requests that do not use the fetch adapter.
- Processes where `Object.prototype` is not polluted.

## Technical Details

`lib/adapters/fetch.js` builds:

```js
const resolvedOptions = {
...fetchOptions,
signal: composedSignal,
method: method.toUpperCase(),
headers: toByteStringHeaderObject(headers.normalize()),
body: data,
duplex: 'half',
credentials: isCredentialsSupported ? withCredentials : undefined,
};

request = isRequestSupported && new Request(url, resolvedOptions);

let response = await (isRequestSupported
? _fetch(request, fetchOptions)
: _fetch(url, resolvedOptions));
```

The fallback path without `Request` uses `resolvedOptions`, but the `Request` path passes the original `fetchOptions` as the second argument to `fetch()`. That second argument can contain inherited properties from `Object.prototype`.

Local verification on axios `1.18.1` set `Object.prototype.headers = { Authorization: 'Bearer POLLUTED' }` and called the fetch adapter with `headers: { 'X-Good': 'yes' }, fetchOptions: {}`. The loopback server received `Authorization: Bearer POLLUTED` and did not receive `X-Good`.

## Proof of Concept of Attack

Constrained local demonstration:

```js
Object.prototype.headers = { Authorization: 'Bearer POLLUTED' };
try {
await axios.get(url, {
adapter: 'fetch',
headers: { 'X-Good': 'yes' },
fetchOptions: {}
});
} finally {
delete Object.prototype.headers;
}
```

Expected safe behavior is that the sanitized axios headers remain in force. Current behavior can use the inherited fetch init headers instead.

## Workarounds

Use the Node HTTP adapter for security-sensitive server-side requests until fixed. If the fetch adapter must be used, avoid passing empty `fetchOptions` objects in processes where prototype pollution is possible, and set explicit safe own values for fetch init fields.

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

Hello, I’m not completely sure if this is something you’d consider a security issue, since it depends on prototype pollution happening somewhere else first, but I wanted to report it just in case.

I was testing the fetch adapter with polluted prototype values and found that `Object.prototype.headers` can change the request axios sends.

The issue seems to be in `lib/adapters/fetch.js`. Axios creates a `Request` with the resolved headers/method/body, but then sends it with `fetch(request, fetchOptions)`. With undici, if `fetchOptions` doesn’t have its own headers, an inherited `Object.prototype.headers` value can be used during the final fetch call.

I tested it with this:

```js
import http from 'node:http';
import axios from 'axios';

const server = http.createServer((req, res) => {
res.end(JSON.stringify({
authorization: req.headers.authorization || null,
xGood: req.headers['x-good'] || null
}));
});

await new Promise(resolve => server.listen(0, '127.0.0.1', resolve));
const { port } = server.address();

Object.prototype.headers = {
Authorization: 'Bearer POLLUTED'
};

try {
const res = await axios.get(`http://127.0.0.1:${port}/`, {
adapter: 'fetch',
headers: { 'X-Good': 'yes' },
fetchOptions: {}
});

console.log(res.data);
} finally {
delete Object.prototype.headers;
server.close();
}
```

The result I get is:

```json
{
"authorization": "Bearer POLLUTED",
"xGood": null
}
```

So the polluted Authorization header is sent, and the normal axios header is not.

Changing the fetch call to pass the already resolved options fixes it for me:

```diff
- _fetch(request, fetchOptions)
+ _fetch(request, resolvedOptions)
```

`resolvedOptions` already includes `...fetchOptions`, so custom fetch options should still work, while headers, method, body, and signal stay as clean own values.
</details>

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