fix(event-handler): fixed CORS behaviour not aligned with CORS spec

packages/event-handler/src/rest/middleware/cors.ts

@@ -1,26 +1,10 @@
-import type {
-  CorsOptions,
-  Middleware,
-} from '../../types/rest.js';
+import type { CorsOptions, Middleware } from '../../types/rest.js';
 import {
   DEFAULT_CORS_OPTIONS,
   HttpErrorCodes,
   HttpVerbs,
 } from '../constants.js';
 
-/**
- * Resolves the origin value based on the configuration
- */
-const resolveOrigin = (
-  originConfig: NonNullable<CorsOptions['origin']>,
-  requestOrigin: string | null,
-): string => {
-  if (Array.isArray(originConfig)) {
-    return requestOrigin && originConfig.includes(requestOrigin) ? requestOrigin : '';
-  }
-  return originConfig;
-};
-
 /**
  * Creates a CORS middleware that adds appropriate CORS headers to responses
  * and handles OPTIONS preflight requests.
@@ -29,9 +13,9 @@ const resolveOrigin = (
  * ```typescript
  * import { Router } from '@aws-lambda-powertools/event-handler/experimental-rest';
  * import { cors } from '@aws-lambda-powertools/event-handler/experimental-rest/middleware';
- * 
+ *
  * const app = new Router();
- * 
+ *
  * // Use default configuration
  * app.use(cors());
  *
@@ -50,7 +34,7 @@ const resolveOrigin = (
  *   }
  * }));
  * ```
- * 
+ *
  * @param options.origin - The origin to allow requests from
  * @param options.allowMethods - The HTTP methods to allow
  * @param options.allowHeaders - The headers to allow
@@ -61,38 +45,93 @@ const resolveOrigin = (
 export const cors = (options?: CorsOptions): Middleware => {
   const config = {
     ...DEFAULT_CORS_OPTIONS,
-    ...options
+    ...options,
   };
+  const allowedOrigins =
+    typeof config.origin === 'string' ? [config.origin] : config.origin;
+  const allowsWildcard = allowedOrigins.includes('*');
+  const allowedMethods = config.allowMethods.map((method) =>
+    method.toUpperCase()
+  );
+  const allowedHeaders = config.allowHeaders.map((header) =>
+    header.toLowerCase()
+  );
 
-  return async (_params, reqCtx, next) => {
-    const requestOrigin = reqCtx.request.headers.get('Origin');
-    const resolvedOrigin = resolveOrigin(config.origin, requestOrigin);
+  const isOriginAllowed = (
+    requestOrigin: string | null
+  ): requestOrigin is string => {
+    return (
+      requestOrigin !== null &&
+      (allowsWildcard || allowedOrigins.includes(requestOrigin))
+    );
+  };
 
-    reqCtx.res.headers.set('access-control-allow-origin', resolvedOrigin);
-    if (resolvedOrigin !== '*') {
-      reqCtx.res.headers.set('Vary', 'Origin');
+  const isValidPreflightRequest = (requestHeaders: Headers) => {
+    const accessControlRequestMethod = requestHeaders
+      .get('Access-Control-Request-Method')
+      ?.toUpperCase();
+    const accessControlRequestHeaders = requestHeaders
+      .get('Access-Control-Request-Headers')
+      ?.toLowerCase();
+    return (
+      accessControlRequestMethod &&
+      allowedMethods.includes(accessControlRequestMethod) &&
+      accessControlRequestHeaders
+        ?.split(',')
+        .every((header) => allowedHeaders.includes(header.trim()))
+    );
+  };
+
+  const setCORSBaseHeaders = (
+    requestOrigin: string,
+    responseHeaders: Headers
+  ) => {
+    const resolvedOrigin = allowsWildcard ? '*' : requestOrigin;
+    responseHeaders.set('access-control-allow-origin', resolvedOrigin);
+    if (!allowsWildcard && Array.isArray(config.origin)) {
+      responseHeaders.set('vary', 'Origin');
     }
-    config.allowMethods.forEach(method => {
-      reqCtx.res.headers.append('access-control-allow-methods', method);
-    });
-    config.allowHeaders.forEach(header => {
-      reqCtx.res.headers.append('access-control-allow-headers', header);
-    });
-    config.exposeHeaders.forEach(header => {
-      reqCtx.res.headers.append('access-control-expose-headers', header);
-    });
-    reqCtx.res.headers.set('access-control-allow-credentials', config.credentials.toString());
-    if (config.maxAge !== undefined) {
-      reqCtx.res.headers.set('access-control-max-age', config.maxAge.toString());
+    if (config.credentials) {
+      responseHeaders.set('access-control-allow-credentials', 'true');
+    }
+  };
+
+  return async (_params, reqCtx, next) => {
+    const requestOrigin = reqCtx.request.headers.get('Origin');
+    if (!isOriginAllowed(requestOrigin)) {
+      await next();
+      return;
     }
 
     // Handle preflight OPTIONS request
-    if (reqCtx.request.method === HttpVerbs.OPTIONS && reqCtx.request.headers.has('Access-Control-Request-Method')) {
+    if (reqCtx.request.method === HttpVerbs.OPTIONS) {
+      if (!isValidPreflightRequest(reqCtx.request.headers)) {
+        await next();
+        return;
+      }
+      setCORSBaseHeaders(requestOrigin, reqCtx.res.headers);
+      if (config.maxAge !== undefined) {
+        reqCtx.res.headers.set(
+          'access-control-max-age',
+          config.maxAge.toString()
+        );
+      }
+      for (const method of allowedMethods) {
+        reqCtx.res.headers.append('access-control-allow-methods', method);
+      }
+      for (const header of allowedHeaders) {
+        reqCtx.res.headers.append('access-control-allow-headers', header);
+      }
       return new Response(null, {
         status: HttpErrorCodes.NO_CONTENT,
         headers: reqCtx.res.headers,
       });
     }
+
+    setCORSBaseHeaders(requestOrigin, reqCtx.res.headers);
+    for (const header of config.exposeHeaders) {
+      reqCtx.res.headers.append('access-control-expose-headers', header);
+    }
     await next();
   };
 };