fix: discover OIDC metadata server-side to avoid CORS failures #382

app/api/auth/oauth/metadata/route.ts

@@ -0,0 +1,53 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { logger } from '@/lib/logger';
+import { configManager } from '@/lib/admin/config-manager';
+import { getMetadata, getRequiredConfig } from '@/lib/oauth/token-exchange';
+
+/**
+ * Same-origin OAuth metadata (discovery) proxy.
+ *
+ * The login page needs the authorization_endpoint to build the PKCE authorize
+ * URL in the browser. Discovering it directly from the browser means a
+ * cross-origin fetch to the IdP's /.well-known/* documents, which is subject
+ * to CORS: providers like Authentik serve those documents without an
+ * Access-Control-Allow-Origin header, so the browser blocks the response and
+ * discovery fails (issue #382). Performing discovery here - server to server,
+ * where CORS does not apply - and handing the result back as a same-origin
+ * response sidesteps the problem entirely.
+ *
+ * The discovery URL is resolved from admin config (via server_id), never from
+ * client input, so this cannot be abused as an open SSRF proxy. Endpoint URLs
+ * in the discovered document are still gated by the SSRF validator inside
+ * discoverOAuth. The returned fields are public well-known metadata.
+ */
+export async function GET(request: NextRequest) {
+  await configManager.ensureLoaded();
+  const serverId = request.nextUrl.searchParams.get('server_id');
+
+  let discoveryUrl: string;
+  try {
+    ({ discoveryUrl } = getRequiredConfig(serverId));
+  } catch {
+    // OAuth not configured for this server - surface as "no metadata" rather
+    // than a 500 so the login page just hides the SSO button.
+    return NextResponse.json({ error: 'OAuth not configured' }, { status: 404 });
+  }
+
+  try {
+    const metadata = await getMetadata(serverId);
+    if (!metadata?.authorization_endpoint || !metadata.token_endpoint) {
+      logger.warn('OAuth metadata discovery returned no usable endpoints', { discoveryUrl });
+      return NextResponse.json({ error: 'OAuth discovery failed' }, { status: 502 });
+    }
+    return NextResponse.json(metadata, {
+      // Mirror the in-process discovery cache TTL so repeated login-page loads
+      // hit the CDN/browser cache instead of re-running discovery.
+      headers: { 'Cache-Control': 'private, max-age=600' },
+    });
+  } catch (error) {
+    logger.error('OAuth metadata discovery error', {
+      error: error instanceof Error ? error.message : 'Unknown error',
+    });
+    return NextResponse.json({ error: 'OAuth discovery failed' }, { status: 502 });
+  }
+}