Axios: ReDoS (O(N²)) in shouldBypassProxy host normalization, reachable via untrusted redirect Location

Not affected:

• Browser adapters.
• Requests with proxy: false.
• Requests with no NO_PROXY value.
• Requests with maxRedirects: 0, unless application code manually follows the malicious redirect and re-enters axios.

Technical Details

lib/helpers/shouldBypassProxy.js contains:

return unmapIPv4MappedIPv6(hostname.replace(/\.+$/, ''));

When the hostname is "." * n + "a", the $ anchor causes the regex engine to retry the dot run from many positions before it fails. setProxy() invokes shouldBypassProxy(location) after getProxyForUrl(location) returns a proxy, including on redirect hops via beforeRedirects.proxy.

Local timing on axios 1.18.1 showed increasing cost for crafted hostnames: about 1 ms at 1000 dots, 6.9 ms at 3000 dots, and 34.5 ms at 6000 dots. The growth is consistent with the submitted quadratic claim while avoiding long-running payloads.

Proof of Concept of Attack

Constrained helper-level demonstration:

import shouldBypassProxy from 'axios/lib/helpers/shouldBypassProxy.js';

process.env.NO_PROXY = 'example.com';
shouldBypassProxy('http://' + '.'.repeat(6000) + 'a/');

In the full adapter path, a malicious server can return that hostname in a 302 Location header while the client has proxy environment variables and NO_PROXY configured.

Workarounds

Disable automatic redirects for requests to untrusted servers, or avoid environment proxy handling for those requests with proxy: false when appropriate. Operators can also avoid broad untrusted redirect-following in services where event-loop availability is critical.

Summary

shouldBypassProxy normalizes a host with hostname.replace(/.+$/, ''). On a host of the shape <many dots><non-dot> (e.g. "." × 40000 + "a"), this anchored regex backtracks quadratically (O(N²)), synchronously starving the Node.js event loop. Because axios re-evaluates the proxy on every redirect using the new Location host, a malicious server can return a crafted 302 Location and freeze the client's event loop for seconds per redirect — a denial of service.

Details

• Affected code: lib/helpers/shouldBypassProxy.js → normalizeNoProxyHost: hostname.replace(/.+$/, '') (one pass in 1.18.1). The open PR #11029 adds a second /.+$/ pass in normalizeIPAddress, doubling the cost (not the origin).
• Root cause: /.+$/ is O(N²) on a long run of dots that is not at the end of the string — the $ anchor forces the engine to backtrack the entire dot-run from every start position.
• Trust boundary (per THREATMODEL): the redirect Location is untrusted (T-2: "axios to network … redirect Location … untrusted"). The caller requests a benign URL; the malicious host arrives via the server's 302. This is not the T-1 caller-supplied-URL non-goal.
• Call chain: setProxy(options, configProxy, location, isRedirect, …) (lib/adapters/http.js) → env-proxy branch → getProxyForUrl(location) returns a proxy → if (!shouldBypassProxy(location)) → normalizeNoProxyHost(parsed.hostname.toLowerCase()) runs /.+$/. setProxy is re-invoked on the redirect hop with the untrusted Location; new URL() retains the long dot-run in .hostname.
• Preconditions: proxy configured via environment (HTTP_PROXY/HTTPS_PROXY, trusted per THREATMODEL T-3, common in CI/containers/enterprise) + NO_PROXY set + redirects followed (default maxRedirects: 5).

PoC

Self-contained, no network — imports axios's own helper: // node poc.mjs (run next to an axios install) import sbp from './node_modules/axios/lib/helpers/shouldBypassProxy.js'; process.env.NO_PROXY = 'example.com'; for (const n of [5000, 10000, 20000, 40000]) { const url = 'http://' + '.'.repeat(n) + 'a/'; // arrives as an untrusted 302 Location host const t0 = process.hrtime.bigint(); sbp(url); // returns false (correct no-bypass) — but O(N^2) slow console.log(N=${n}: ${(Number(process.hrtime.bigint()-t0)/1e6)|0} ms); } // Measured on 1.18.1: N=5000 ~43ms, 10000 ~160ms, 20000 ~618ms, 40000 ~2499ms; benign host ~0.05ms. Driven through the real http-adapter __setProxy on the redirect path (setProxy(…, isRedirect=true)), a 10 ms timer fires 0 times during the ~2.4 s block at N=40000 — full event-loop starvation.

Impact

Denial of service (event-loop starvation) on any axios client that uses an environment proxy with NO_PROXY set and follows redirects, when a server it contacts returns a crafted redirect Location. No data exposure or RCE. Same impact class as the accepted DoS advisories GHSA-62hf-57xw-28j9 (toFormData recursion) and the maxContentLength response-size DoS.

Suggested fix

Replace the regex trailing-dot strip with a linear trim: let end = hostname.length; while (end > 0 && hostname.charCodeAt(end - 1) === 46 /* '.' */) end--; hostname = hostname.slice(0, end); Also drop the redundant second /.+$/ pass in PR #11029's normalizeIPAddress.