fix: secure authentication request identity

Replace Better Auth's default trust of a caller-controlled X-Forwarded-For
with a platform-owned client-address resolution boundary: direct mode (the
safe default) trusts only the real socket peer, and an explicit trusted-proxy
mode requires a header name, a concrete trusted-proxy list, and only honors a
forwarded chain from an actual trusted peer. This closes a rate-limit bypass
where rotating a spoofed header let a caller choose a fresh bucket on every
request. Public sign-up is also disabled by default and requires an explicit
typed opt-in, using Better Auth's own disableSignUp gate so no email-existence
leak or second signup path is introduced.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

apps/api/src/client-address.ts

@@ -0,0 +1,66 @@
+import { type ClientAddressResolver, TRUSTED_CLIENT_ADDRESS_HEADER } from "@ekklesia/core/identity";
+import type { Http2Bindings, HttpBindings } from "@hono/node-server";
+
+type NodeServerBindings = HttpBindings | Http2Bindings;
+
+export {
+  type ClientAddressConfig,
+  ClientAddressConfigError,
+  type ClientAddressConfigErrorCode,
+  type ClientAddressConfigInput,
+  type ClientAddressFailureReason,
+  type ClientAddressMode,
+  type ClientAddressResolution,
+  type ClientAddressResolver,
+  createClientAddressConfig,
+  createClientAddressResolver,
+  type DirectClientAddressConfig,
+  findInvalidTrustedProxies,
+  normalizeClientAddress,
+  type TrustedProxyClientAddressConfig,
+} from "@ekklesia/core/identity";
+
+/** A fixed, generic diagnostic — never the peer address, forwarded header content, request path, or
+ * any other request detail — logged only when client-address resolution fails closed. */
+export const CLIENT_ADDRESS_UNRESOLVED_LOG_MESSAGE =
+  "[client-address] A request's caller address could not be resolved safely; rejecting with 503.";
+
+function sanitizedClientAddressUnresolvedResponse(): Response {
+  return new Response(JSON.stringify({ code: "CLIENT_ADDRESS_UNRESOLVED" }), {
+    headers: { "cache-control": "no-store", "content-type": "application/json" },
+    status: 503,
+  });
+}
+
+/**
+ * Wraps a `@hono/node-server` fetch callback at the exact Node/API transport boundary — the one
+ * place in this repository below `apps/api` that reads the raw Node `IncomingMessage` — with
+ * Core's framework-agnostic client-address resolution policy (`@ekklesia/core/identity`). Resolves
+ * the caller's trusted address from the real socket peer (`env.incoming.socket.remoteAddress`)
+ * before the wrapped callback — and therefore Better Auth's own rate limiter — ever sees the
+ * request, then overwrites (never merely appends) any externally supplied copy of
+ * `TRUSTED_CLIENT_ADDRESS_HEADER` with that resolved value. A caller can therefore never choose its
+ * own resolved address, whether by spoofing a forwarded header or by sending the internal header
+ * directly. Resolution failure never reaches the wrapped callback at all — no Better Auth logic, no
+ * Core mutation, and no module handler ever runs for a request whose caller address could not be
+ * resolved safely.
+ */
+export function wrapFetchWithClientAddress(
+  fetchCallback: (request: Request, env: NodeServerBindings) => Response | Promise<Response>,
+  resolver: ClientAddressResolver,
+): (request: Request, env: NodeServerBindings) => Promise<Response> {
+  return async (request: Request, env: NodeServerBindings): Promise<Response> => {
+    const resolution = resolver.resolve({
+      headers: request.headers,
+      peerAddress: env.incoming.socket.remoteAddress,
+    });
+    if (!resolution.ok) {
+      console.error(CLIENT_ADDRESS_UNRESOLVED_LOG_MESSAGE);
+      return sanitizedClientAddressUnresolvedResponse();
+    }
+    const headers = new Headers(request.headers);
+    headers.set(TRUSTED_CLIENT_ADDRESS_HEADER, resolution.address);
+    const sanitizedRequest = new Request(request, { headers });
+    return fetchCallback(sanitizedRequest, env);
+  };
+}