axios's shouldBypassProxy does not recognize IPv4-mapped IPv6 addresses, allowing NO_PROXY bypass (incomplete fix for CVE-2025-62718)
Summary
shouldBypassProxy, introduced in v1.15.0 to fix CVE-2025-62718, does not normalise IPv4-mapped IPv6 addresses. When NO_PROXY lists an IPv4 address such as 127.0.0.1 or 169.254.169.254, a request URL using the IPv4-mapped IPv6 form (::ffff:7f00:1, ::ffff:a9fe:a9fe) still routes through the configured proxy. Node.js resolves these addresses to the underlying IPv4 host, so the request reaches the internal service via the proxy rather than being blocked.
Details
lib/helpers/shouldBypassProxy.js (v1.15.0):
```javascript
const LOOPBACK_ADDRESSES = new Set(['localhost', '127.0.0.1', '::1']);
const isLoopback = (host) => LOOPBACK_ADDRESSES.has(host);
// normalizeNoProxyHost strips brackets and trailing dots, but not ::ffff: prefix
return hostname === entryHost || (isLoopback(hostname) && isLoopback(entryHost));
```
The WHATWG URL parser canonicalises http://[::ffff:127.0.0.1]/ to hostname [::ffff:7f00:1]. After bracket-stripping: ::ffff:7f00:1. This string does not match 127.0.0.1 in NO_PROXY and is not in LOOPBACK_ADDRESSES, so shouldBypassProxy returns false and the proxy is used. proxy-from-env (called before shouldBypassProxy) has the same gap - it does not equate ::ffff:7f00:1 with 127.0.0.1 - so neither layer catches the bypass.
PoC
```javascript
// NO_PROXY=127.0.0.1,localhost,::1 HTTP_PROXY=http://attacker:8080
import shouldBypassProxy from 'axios/lib/helpers/shouldBypassProxy.js';
// All three should return true (bypass proxy). Only the first two do.
console.log(shouldBypassProxy('http://127.0.0.1/')); // true [OK]
console.log(shouldBypassProxy('http://[::1]/')); // true [OK]
console.log(shouldBypassProxy('http://[::ffff:127.0.0.1]/')); // false <- bypass
console.log(shouldBypassProxy('http://[::ffff:7f00:1]/')); // false <- bypass
```
Node.js routes ::ffff:7f00:1 to 127.0.0.1:
```
// net.connect({ host: '::ffff:7f00:1', port: 80 }) reaches a service
// bound to 127.0.0.1:80 — confirmed on Node.js v24, Linux and macOS.
```
Cloud metadata SSRF: ::ffff:a9fe:a9fe = ::ffff:169.254.169.254. If NO_PROXY=169.254.169.254 is set to block IMDS access, a request
