fix: optimize cores headers (#5629)

This commit enhances two aspects of CORS:

- Always support CORS preflight requests.
- Do not add additional secuity headers for prefligh calls.

src/lib/app.ts

@@ -102,10 +102,7 @@ export default async function getApp(
     // so this must be handled before the API token middleware.
     app.options(
         `${baseUriPath}/api/frontend*`,
-        conditionalMiddleware(
-            () => config.flagResolver.isEnabled('embedProxy'),
-            corsOriginMiddleware(services, config),
-        ),
+        corsOriginMiddleware(services, config),
     );
 
     app.use(baseUriPath, patMiddleware(config, services));

src/lib/middleware/conditional-middleware.ts

@@ -7,7 +7,6 @@ export const conditionalMiddleware = (
     const router = Router();
 
     router.use((req, res, next) => {
-        res.setHeader('Vary', 'Origin');
         if (condition()) {
             middleware(req, res, next);
         } else {

src/lib/middleware/cors-origin-middleware.ts

@@ -19,7 +19,7 @@ export const corsOriginMiddleware = (
     { proxyService }: Pick<IUnleashServices, 'proxyService'>,
     config: IUnleashConfig,
 ): RequestHandler => {
-    return cors(async (req, callback) => {
+    const corsFunc = cors(async (req, callback) => {
         try {
             const { frontendApiOrigins = [] } =
                 await proxyService.getFrontendSettings();
@@ -33,4 +33,8 @@ export const corsOriginMiddleware = (
             callback(error);
         }
     });
+    return (req, res, next) => {
+        res.setHeader('Vary', 'Origin');
+        corsFunc(req, res, next);
+    };
 };

src/lib/middleware/secure-headers.ts

@@ -116,11 +116,11 @@ const secureHeaders: (config: IUnleashConfig) => RequestHandler = (config) => {
         });
 
         return (req, res, next) => {
-            const stripHeadersOnAPI =
-                config.flagResolver.isEnabled('stripHeadersOnAPI');
-            if (
+            if (req.method === 'OPTIONS') {
+                return next();
+            } else if (
                 req.path.startsWith(`${config.server.baseUriPath}/api/`) &&
-                stripHeadersOnAPI
+                config.flagResolver.isEnabled('stripHeadersOnAPI')
             ) {
                 apiHelmet(req, res, next);
             } else {