Allocation of Resources Without Limits or Throttling in Axios

Summary

Axios versions 1.7.0 through 1.15.x did not enforce configured request and response size limits when requests were sent with the fetch adapter. Applications that selected adapter: 'fetch', or ran in environments where axios resolved to the fetch adapter, could receive or send bodies larger than maxContentLength or maxBodyLength despite those limits being explicitly configured.

This can cause resource exhaustion in server-side usage when a malicious or compromised server returns an oversized response, when an attacker can supply a large data: URL, or when an application forwards attacker-controlled request bodies through axios while relying on maxBodyLength as a boundary.

Impact

The impact is availability-only. Affected applications may process, buffer, or transmit data beyond the configured limit, potentially exhausting memory, CPU, or network resources.

This does not affect axios’s default unlimited behaviour by itself: maxContentLength and maxBodyLength default to -1. The vulnerability exists when an application has configured finite limits and expects axios to enforce them.

Server-side runtimes are the primary concern. Browser impact is generally constrained by the browser process and browser fetch behavior, and should not be described as server process exhaustion.

Affected Functionality

Affected functionality includes requests using the built-in fetch adapter with finite maxContentLength or maxBodyLength values.

Relevant configurations include:

• adapter: 'fetch'
• adapter: ['fetch', ...] when fetch is selected
• environments where neither xhr nor http is available and axios falls back to fetch
• custom fetch environments configured through env.fetch

Unaffected functionality includes:

• Node.js default http adapter enforcement
• versions before the fetch adapter was introduced
• configurations that do not rely on finite axios size limits

Technical Details

In vulnerable versions, lib/adapters/fetch.js destructured request config without maxContentLength or maxBodyLength. The adapter dispatched fetch() and then materialized the response through text(), arrayBuffer(), blob(), or related resolvers without checking the configured response limit.

The fix in e5540dc added:

• maxContentLength and maxBodyLength reads in lib/adapters/fetch.js
• upfront data: URL decoded-size checks
• outbound body-size checks before dispatch
• Content-Length response pre-checks
• streaming response enforcement
• fallback checks for environments without ReadableStream
• regression tests in tests/unit/adapters/fetch.test.js

Proof of Concept of Attack

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

const server = http.createServer((req, res) => {
  let received = 0;

  req.on('data', chunk => {
    received += chunk.length;
  });

  req.on('end', () => {
    res.end(JSON.stringify({ received }));
  });
});

await new Promise(resolve => server.listen(0, resolve));
const url = `http://127.0.0.1:${server.address().port}/`;

await axios.post(url, 'A'.repeat(2 * 1024 * 1024), {
  adapter: 'fetch',
  maxBodyLength: 1024
});

// Vulnerable versions succeed and the server receives 2097152 bytes.
// Fixed versions reject with ERR_BAD_REQUEST.

server.close();

Workarounds

Use the Node.js http adapter for server-side requests where finite size limits are security-relevant.

Validate or cap attacker-controlled request bodies before passing them to axios.

Reject or strictly allowlist attacker-controlled URL schemes, especially data: URLs, before calling axios.

Summary

When Axios is used with adapter: 'fetch', configured body/response size limits are not enforced. This allows oversized uploads/downloads (including data: URLs) despite explicit limits, which can lead to memory/resource exhaustion in server-side usage.

Details

maxBodyLength and maxContentLength are not applied in the fetch adapter flow:

• lib/adapters/fetch.js (146-160): config destructuring does not include these controls.
• lib/adapters/fetch.js (220-234): request is dispatched with fetch() without request-size enforcement.
• lib/adapters/fetch.js (267-283): response is materialized via text(), arrayBuffer(), blob(), etc. without response-size checks. By contrast, the HTTP adapter enforces both limits.

PoC

Environment:

• Axios main at commit f7a4ee2
• Node v24.2.0

Steps:

1. Start an HTTP server that counts received bytes and echoes {received}.
2. Send 2 MiB with:
   • adapter: 'fetch'
   • maxBodyLength: 1024
3. Request a 4 KiB data: URL with:
   • adapter: 'fetch'
   • maxContentLength: 16

Expected secure behavior: both requests rejected. Observed:

• Upload: success, server received 2097152
• data: response: success, length 4096

Impact

Type: DoS / resource exhaustion due to limit bypass. Impacted: applications using Axios fetch adapter as a server-side security control boundary for untrusted request/response sizes.