Axios: ReDoS in fromDataURI data: URL parser freezes the Node event loop (DoS)

Summary

Axios for Node.js parses data: URLs in lib/helpers/fromDataURI.js. The current RFC-2397 parser uses a regular expression whose media type groups allow / inside both sides of the type/subtype match. A malformed data: URL containing many slashes and no comma forces the JavaScript regex engine to try many possible placements for the separator before failing.

Applications are affected when they pass untrusted URL strings to axios and do not reject or constrain data: URLs before axios parses them.

Impact

An attacker can make the Node.js event loop spend significant synchronous CPU time parsing a single malformed URL. In a server that accepts a URL from an HTTP request and calls axios.get(url), this can block unrelated requests and health checks until parsing completes.

The issue is availability-only. It does not disclose data or modify requests.

Affected Functionality

Affected:

• Node.js HTTP adapter data URL handling.
• axios.get() or equivalent calls where config.url has the data: protocol.

Not affected:

• Browser fetch/XHR URL handling.
• Node requests where the application rejects data: URLs before calling axios.
• Older checked 0.x data URL parser shape unless separately proven vulnerable.

Technical Details

lib/helpers/fromDataURI.js contains:

const DATA_URL_PATTERN = /^([^,;]+\/[^,;]+)?((?:;[^,;=]+=[^,;]+)*)(;base64)?,([\s\S]*)$/;

The [^,;]+ groups include /, so a long string of slashes without a comma can be partitioned around the required \/ in many ways before the match fails. The match runs before axios can apply request timeout behavior, so timeout does not mitigate the parsing pause.

Local timing on axios 1.18.1 with small payloads showed about 2.4 ms at 1000 slashes, 16.6 ms at 3000 slashes, and 71.5 ms at 6000 slashes, consistent with the submitted quadratic scaling while avoiding long-running payloads.

Proof of Concept of Attack

Constrained helper-level demonstration:

import axios from 'axios';

await axios.get('data:' + '/'.repeat(6000));

The request fails after parsing, but the failure is delayed by synchronous regex work. Larger payloads increase the pause substantially.

Workarounds

Reject data: URLs before passing untrusted input to axios, or enforce a strict maximum URL length for URL-fetching endpoints. Applications that do not need data: URL support should deny that protocol explicitly.

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

ReDoS in fromDataURI data: URL parser freezes the event loop (DoS)

Affected

• Package: axios (Node.js http adapter)
• Versions: 1.x (regex present on v1.x, current release line)
• File: lib/helpers/fromDataURI.js
• CWE-1333 (Inefficient Regular Expression Complexity)

Issue

fromDataURI parses data: URLs with this regex:

// lib/helpers/fromDataURI.js:9
const DATA_URL_PATTERN = /^([^,;]+\/[^,;]+)?((?:;[^,;=]+=[^,;]+)*)(;base64)?,([\s\S]*)$/;

The mediatype tokens [^,;]+ include /, so they can span multiple slashes ambiguously. A data: URL made of many slashes with no comma forces the engine to try every way to place the single \/ divider before failing — quadratic O(n²) backtracking. It runs synchronously on the main thread, so the whole Node event loop is frozen for the entire parse. Reachable through the public API on the Node http adapter (axios.get(url)); the browser fetch/xhr adapters are not affected because they do not call fromDataURI.

A configured timeout does not help: the freeze happens during parsing, before any network timer can fire.

PoC (minimal)

import axios from 'axios';
// ~256 KB data: URL of pure slashes, no comma
await axios.get('data:' + '/'.repeat(262139)); // blocks the event loop ~6 min, then throws

Lab results

Single-threaded "fetch a user-supplied URL" service (link-preview style) calling axios.get on a JSON body { "url": "..." }, with timeout: 1000 set.

Scaling is clean O(n²) (constant k ≈ 5.6e-6 ms/byte², stable across sizes):

During the freeze the server answers nothing: a /healthz liveness probe times out for the whole window, and the server's own event-loop monitor cannot even log until the parse finishes. In a run through an intercepting proxy, the proxy hit its 120 s upstream timeout and gave up, while the origin stayed pegged at 100% CPU on one core past that — client/proxy timeouts do not mitigate it.

The attacker controls only the URL string and needs no auth. ~256 KB every ~6 min (≈ 0.7 bytes/sec) keeps a server permanently unavailable.

Impact

Unauthenticated remote denial of service. One small request takes a Node service fully offline for minutes; a trickle keeps it down indefinitely. CVSS 3.1: 7.5 (High) — AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

Fix

RFC 2045 type/subtype tokens never contain /. Excluding / from those two character classes removes the ambiguity and the backtracking:

const DATA_URL_PATTERN = /^([^,;/]+\/[^,;/]+)?((?:;[^,;=]+=[^,;]+)*)(;base64)?,([\s\S]*)$/;

Worst-case parse drops from ~2600 ms to ~0.002 ms. All valid data: URLs, including slashes in the body, parse identically.

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