security: validate Planner callback URL to close an SSRF sink

The session callbackUrl becomes a server-side fetch target in
api.save-to-journal (POSTed with the callback bearer token, and the
journal's response is reflected to the caller). The /new query-param
loader already validated it, but the programmatic POST /api/sessions
entry point — anonymous, since the Planner is stateless — stored it
unvalidated. An attacker could make the Planner backend POST to
arbitrary hosts, including 169.254.169.254 and other internal targets.

- validateFetchUrl now blocks private / loopback / link-local /
  CGNAT / cloud-metadata hosts (IPv4, IPv6, IPv4-mapped) when an
  explicit allowlist isn't set. Gated on NODE_ENV=production && !E2E
  (the requireSecret idiom) so the dev/e2e journal-on-localhost save
  flow is unaffected. An explicit PLANNER_CALLBACK_ALLOWED_HOSTS still
  takes precedence and remains the full-closure control (it also stops
  DNS-name-to-private rebinding, which literal blocking does not).
- POST /api/sessions now validates callbackUrl exactly as /new does.
- api.save-to-journal re-validates immediately before the fetch
  (defense in depth: covers sessions persisted before this change and
  narrows the create→save rebinding window).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/planner/app/lib/url-validation.server.ts

@@ -16,6 +16,60 @@ export interface UrlValidationResult {
   url?: URL;
 }
 
+/**
+ * Whether to reject callback hosts pointing at private / loopback /
+ * link-local / cloud-metadata ranges. Active in real production only —
+ * mirrors the `requireSecret` guard in the journal's config.server.ts.
+ * Dev and E2E legitimately point the callback at the journal on
+ * `localhost`, so blocking there would break the save flow.
+ */
+function blockPrivateAddresses(): boolean {
+  return process.env.NODE_ENV === "production" && process.env.E2E !== "true";
+}
+
+function isBlockedIpv4(ip: string): boolean {
+  const o = ip.split(".").map((n) => Number(n));
+  // Malformed dotted-quad → treat as blocked (fail safe).
+  if (o.length !== 4 || o.some((n) => !Number.isInteger(n) || n < 0 || n > 255)) {
+    return true;
+  }
+  const [a, b] = o as [number, number, number, number];
+  if (a === 0 || a === 10 || a === 127) return true; // this-host, RFC1918 /8, loopback
+  if (a === 169 && b === 254) return true; // link-local incl. 169.254.169.254 metadata
+  if (a === 172 && b >= 16 && b <= 31) return true; // RFC1918 /12
+  if (a === 192 && b === 168) return true; // RFC1918 /16
+  if (a === 100 && b >= 64 && b <= 127) return true; // CGNAT 100.64/10
+  return false;
+}
+
+/**
+ * Literal private/loopback/link-local hosts. Catches IP literals and
+ * `localhost`; it does NOT resolve DNS, so a public name that resolves
+ * to a private IP (DNS-rebinding-style SSRF) is not blocked here — set
+ * PLANNER_CALLBACK_ALLOWED_HOSTS to close that where the callback host
+ * is known ahead of time.
+ */
+function isBlockedHost(hostname: string): boolean {
+  // Node keeps IPv6 brackets on URL.hostname (e.g. "[::1]"); strip them.
+  const h = hostname.toLowerCase().replace(/^\[|\]$/g, "");
+  if (h === "localhost" || h.endsWith(".localhost")) return true;
+  if (h.includes(":")) {
+    // IPv6.
+    if (h === "::1" || h === "::") return true;
+    if (h.startsWith("fc") || h.startsWith("fd")) return true; // ULA fc00::/7
+    if (h.startsWith("fe8") || h.startsWith("fe9") || h.startsWith("fea") || h.startsWith("feb")) {
+      return true; // link-local fe80::/10
+    }
+    // IPv4-mapped (::ffff:a.b.c.d, which Node may render in hex form).
+    // Reaching IPv4 through a mapped address is inherently suspect for a
+    // callback target, so block the whole prefix.
+    if (h.startsWith("::ffff:")) return true;
+    return false;
+  }
+  if (/^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/.test(h)) return isBlockedIpv4(h);
+  return false;
+}
+
 /**
  * Validate a URL string for use as a fetch target. Requires absolute
  * http(s) URL. If `allowedHosts` is provided, the host must be on the
@@ -39,9 +93,16 @@ export function validateFetchUrl(
     return { ok: false, reason: `disallowed scheme ${parsed.protocol}` };
   }
   if (opts.allowedHosts && opts.allowedHosts.length > 0) {
+    // An explicit allowlist is the operator's decision and the strongest
+    // control; a host that matches it is trusted even if it's private.
     if (!opts.allowedHosts.includes(parsed.host)) {
       return { ok: false, reason: `host ${parsed.host} not on allowlist` };
     }
+  } else if (blockPrivateAddresses() && isBlockedHost(parsed.hostname)) {
+    return {
+      ok: false,
+      reason: `host ${parsed.hostname} is a private/loopback/link-local address`,
+    };
   }
   return { ok: true, url: parsed };
 }