fix: validate URLs before outbound fetch

lib/security/url-guard.ts

@@ -0,0 +1,67 @@
+import { lookup } from 'node:dns/promises';
+import { BlockList, isIP } from 'node:net';
+
+const blockedAddressRanges = new BlockList();
+blockedAddressRanges.addAddress('0.0.0.0');
+blockedAddressRanges.addAddress('127.0.0.1');
+blockedAddressRanges.addSubnet('10.0.0.0', 8);
+blockedAddressRanges.addSubnet('172.16.0.0', 12);
+blockedAddressRanges.addSubnet('192.168.0.0', 16);
+blockedAddressRanges.addSubnet('169.254.0.0', 16);
+blockedAddressRanges.addAddress('::', 'ipv6');
+blockedAddressRanges.addAddress('::1', 'ipv6');
+blockedAddressRanges.addSubnet('fc00::', 7, 'ipv6');
+blockedAddressRanges.addSubnet('fe80::', 10, 'ipv6');
+
+const BLOCKED_HOSTNAMES = new Set(['localhost']);
+const BLOCKED_HOSTNAME_SUFFIXES = ['.localhost', '.local', '.internal', '.arpa', '.localdomain'];
+
+function normalizeHostname(hostname: string): string {
+  return hostname.replace(/^\[(.*)\]$/, '$1').toLowerCase();
+}
+
+function isBlockedIpAddress(hostname: string): boolean {
+  const normalized = normalizeHostname(hostname);
+  const family = isIP(normalized);
+  if (family === 4) return blockedAddressRanges.check(normalized, 'ipv4');
+  if (family === 6) return blockedAddressRanges.check(normalized, 'ipv6');
+  return false;
+}
+
+/**
+ * Returns true only when the URL targets a public host reachable over http(s).
+ * Rejects loopback / RFC-1918 / link-local / ULA addresses, special hostname
+ * suffixes (.local, .internal, .arpa, ...), URLs with embedded credentials,
+ * and any hostname whose DNS resolves to a blocked address.
+ *
+ * Note: there is a TOCTOU window between this lookup and the eventual fetch().
+ * Callers that need rebinding-safe behavior must additionally pin the resolved
+ * IP at connect time (e.g. via a custom undici dispatcher).
+ */
+export async function isPublicHttpUrl(urlString: string): Promise<boolean> {
+  let url: URL;
+  try {
+    url = new URL(urlString);
+  } catch {
+    return false;
+  }
+
+  if (url.protocol !== 'http:' && url.protocol !== 'https:') return false;
+  if (url.username || url.password) return false;
+
+  const hostname = normalizeHostname(url.hostname);
+  if (!hostname) return false;
+  if (BLOCKED_HOSTNAMES.has(hostname)) return false;
+  if (BLOCKED_HOSTNAME_SUFFIXES.some((suffix) => hostname.endsWith(suffix))) return false;
+
+  if (isBlockedIpAddress(hostname)) return false;
+  if (isIP(hostname)) return true;
+
+  try {
+    const records = await lookup(hostname, { all: true, verbatim: true });
+    if (records.length === 0) return false;
+    return records.every((record) => !isBlockedIpAddress(record.address));
+  } catch {
+    return false;
+  }
+}