fix: implement dynamic CORS headers based on request origins (Fixes #1049)

workers/well-known-cache/src/index.ts

@@ -5,12 +5,33 @@ interface Env {
   STELLAR_TOML_STALE_WHILE_REVALIDATE: string;
   DEFAULT_MAX_AGE: string;
   DEFAULT_STALE_WHILE_REVALIDATE: string;
+  ALLOWED_ORIGINS: string;
 }
+function parseAllowedOrigins(raw: string): Set<string> {
+  return new Set(
+    raw
+      .split(",")
+      .map((s) => s.trim())
+      .filter(Boolean),
+  );
+}
+
+function getCorsHeaders(
+  requestOrigin: string | null,
+  allowedOrigins: Set<string>,
+): Record<string, string> {
+  const headers: Record<string, string> = {
+    "Access-Control-Allow-Methods": "GET, HEAD, OPTIONS",
+    "Access-Control-Allow-Headers": "Content-Type",
+  };
 
-const CORS_HEADERS: Record<string, string> = {
-  "Access-Control-Allow-Origin": "*",
-  "Access-Control-Allow-Methods": "GET, HEAD, OPTIONS",
-  "Access-Control-Allow-Headers": "Content-Type",
+  if (requestOrigin && allowedOrigins.has(requestOrigin)) {
+    headers["Access-Control-Allow-Origin"] = requestOrigin;
+    headers["Vary"] = "Origin";
+  }
+
+  return headers;
+}
 };
 
 interface ErrorResponse {
@@ -20,7 +41,7 @@ interface ErrorResponse {
   timestamp: string;
 }
 
-function errorResponse(status: number, error: string, message: string): Response {
+function errorResponse(status: number, error: string, message: string, corsHeaders: Record<string, string> = {}): Response {
   const body: ErrorResponse = {
     status,
     error,
@@ -31,7 +52,7 @@ function errorResponse(status: number, error: string, message: string): Response
     status,
     headers: {
       "Content-Type": "application/json",
-      ...CORS_HEADERS,
+      ...corsHeaders,
     },
   });
 }
@@ -65,23 +86,28 @@ function logMetrics(metrics: RequestMetrics): void {
 
 export default {
   async fetch(request: Request, env: Env): Promise<Response> {
+    const allowedOrigins = parseAllowedOrigins(env.ALLOWED_ORIGINS ?? "");
+    const requestOrigin = request.headers.get("Origin");
+    const corsHeaders = getCorsHeaders(requestOrigin, allowedOrigins);
+
     if (request.method === "OPTIONS") {
-      return new Response(null, { status: 204, headers: CORS_HEADERS });
+      return new Response(null, { status: 204, headers: corsHeaders });
     }
 
     if (!["GET", "HEAD"].includes(request.method)) {
       return errorResponse(
         405,
         "Method Not Allowed",
-        `HTTP method ${request.method} is not supported. Use GET or HEAD.`
+        `HTTP method ${request.method} is not supported. Use GET or HEAD.`,
+        corsHeaders
       );
     }
 
     let url: URL;
     try {
       url = new URL(request.url);
     } catch {
-      return errorResponse(400, "Bad Request", "Invalid request URL.");
+      return errorResponse(400, "Bad Request", "Invalid request URL.", corsHeaders);
     }
 
     try {
@@ -91,7 +117,7 @@ export default {
       if (cached) {
         const res = new Response(cached.body, cached);
         res.headers.set("cf-cache-status", "HIT");
-        for (const [k, v] of Object.entries(CORS_HEADERS)) res.headers.set(k, v);
+        for (const [k, v] of Object.entries(corsHeaders)) res.headers.set(k, v);
         return res;
       }
 
@@ -100,20 +126,21 @@ export default {
         return errorResponse(
           origin.status,
           origin.statusText || "Upstream Error",
-          `Origin server returned ${origin.status} for ${url.pathname}.`
+          `Origin server returned ${origin.status} for ${url.pathname}.`,
+          corsHeaders
         );
       }
 
       const res = new Response(origin.body, origin);
       res.headers.set("Cache-Control", cacheControlFor(url.pathname));
       res.headers.set("cf-cache-status", "MISS");
-      for (const [k, v] of Object.entries(CORS_HEADERS)) res.headers.set(k, v);
+      for (const [k, v] of Object.entries(corsHeaders)) res.headers.set(k, v);
 
       await cache.put(request, res.clone());
       return res;
     } catch (err) {
       const message = err instanceof Error ? err.message : "An unexpected error occurred.";
-      return errorResponse(502, "Bad Gateway", `Failed to fetch origin: ${message}`);
+      return errorResponse(502, "Bad Gateway", `Failed to fetch origin: ${message}`, corsHeaders);
     }
   },
 } satisfies ExportedHandler<Env>;

wrangler.toml

@@ -14,3 +14,5 @@ STELLAR_TOML_STALE_WHILE_REVALIDATE = "86400"
 # Cache TTL for other .well-known paths (seconds)
 DEFAULT_MAX_AGE = "300"
 DEFAULT_STALE_WHILE_REVALIDATE = "3600"
+# Comma-separated list of allowed CORS origins (exact match)
+ALLOWED_ORIGINS = "https://yourdomain.com"