everything: safer CORS + loopback bind for HTTP transports

src/everything/README.md

@@ -70,6 +70,12 @@ npm install
 npm run start:sse
 ```
 
+Note: the SSE transport binds to `127.0.0.1` by default and only enables CORS for loopback origins (for Inspector direct connect). To intentionally expose it, set:
+
+```shell
+HOST=0.0.0.0 MCP_CORS_ORIGIN_REGEX='^https?://your-allowed-origin(:\\d+)?$' npm run start:sse
+```
+
 ## Run from source with [Streamable HTTP Transport](https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http)
 
 ```shell
@@ -78,6 +84,12 @@ npm install
 npm run start:streamableHttp
 ```
 
+Note: the Streamable HTTP transport binds to `127.0.0.1` by default and only enables CORS for loopback origins (for Inspector direct connect). To intentionally expose it, set:
+
+```shell
+HOST=0.0.0.0 MCP_CORS_ORIGIN_REGEX='^https?://your-allowed-origin(:\\d+)?$' npm run start:streamableHttp
+```
+
 ## Running as an installed package
 ### Install 
 ```shell
@@ -103,4 +115,3 @@ npx @modelcontextprotocol/server-everything sse
 ```shell
 npx @modelcontextprotocol/server-everything streamableHttp
 ```
-

src/everything/transports/cors.ts

@@ -0,0 +1,38 @@
+import type { CorsOptions } from "cors";
+
+const DEFAULT_LOOPBACK_ORIGIN_REGEX =
+  /^https?:\/\/(localhost|127\.0\.0\.1|\[::1\])(?::\d+)?$/;
+
+function getCorsOriginRegex(): RegExp {
+  const raw = process.env.MCP_CORS_ORIGIN_REGEX;
+  if (!raw) return DEFAULT_LOOPBACK_ORIGIN_REGEX;
+
+  try {
+    return new RegExp(raw);
+  } catch (err) {
+    // Fail fast with a clear message instead of silently allowing all origins.
+    throw new Error(
+      `Invalid MCP_CORS_ORIGIN_REGEX=${JSON.stringify(raw)}: ${String(err)}`
+    );
+  }
+}
+
+export function createCorsOptions(opts: {
+  methods: string;
+  exposedHeaders?: string[];
+}): CorsOptions {
+  const originRegex = getCorsOriginRegex();
+
+  return {
+    // Only allow loopback origins by default (Inspector direct connect).
+    // Override via MCP_CORS_ORIGIN_REGEX if you intentionally need a wider allowlist.
+    origin: (origin, callback) => {
+      if (!origin) return callback(null, false);
+      return callback(null, originRegex.test(origin));
+    },
+    methods: opts.methods,
+    preflightContinue: false,
+    optionsSuccessStatus: 204,
+    ...(opts.exposedHeaders ? { exposedHeaders: opts.exposedHeaders } : {}),
+  };
+}

src/everything/transports/sse.ts

@@ -2,18 +2,19 @@ import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
 import express from "express";
 import { createServer } from "../server/index.js";
 import cors from "cors";
+import { createCorsOptions } from "./cors.js";
 
 console.error("Starting SSE server...");
 
-// Express app with permissive CORS for testing with Inspector direct connect mode
+// Express app with loopback-only CORS by default for Inspector direct connect mode.
+// Override via MCP_CORS_ORIGIN_REGEX if you intentionally need a wider allowlist.
 const app = express();
 app.use(
-  cors({
-    origin: "*", // use "*" with caution in production
-    methods: "GET,POST",
-    preflightContinue: false,
-    optionsSuccessStatus: 204,
-  })
+  cors(
+    createCorsOptions({
+      methods: "GET,POST",
+    })
+  )
 );
 
 // Map sessionId to transport for each client
@@ -71,7 +72,11 @@ app.post("/message", async (req, res) => {
 });
 
 // Start the express server
-const PORT = process.env.PORT || 3001;
-app.listen(PORT, () => {
-  console.error(`Server is running on port ${PORT}`);
+const PORT = process.env.PORT ? Number(process.env.PORT) : 3001;
+if (!Number.isFinite(PORT)) {
+  throw new Error(`Invalid PORT=${JSON.stringify(process.env.PORT)}`);
+}
+const HOST = process.env.HOST || "127.0.0.1";
+app.listen(PORT, HOST, () => {
+  console.error(`Server is running on http://${HOST}:${PORT}`);
 });

src/everything/transports/streamableHttp.ts

@@ -6,6 +6,7 @@ import express, { Request, Response } from "express";
 import { createServer } from "../server/index.js";
 import { randomUUID } from "node:crypto";
 import cors from "cors";
+import { createCorsOptions } from "./cors.js";
 
 // Simple in-memory event store for SSE resumability
 class InMemoryEventStore implements EventStore {
@@ -38,16 +39,20 @@ class InMemoryEventStore implements EventStore {
 
 console.log("Starting Streamable HTTP server...");
 
-// Express app with permissive CORS for testing with Inspector direct connect mode
+// Express app with loopback-only CORS by default for Inspector direct connect mode.
+// Override via MCP_CORS_ORIGIN_REGEX if you intentionally need a wider allowlist.
 const app = express();
 app.use(
-  cors({
-    origin: "*", // use "*" with caution in production
-    methods: "GET,POST,DELETE",
-    preflightContinue: false,
-    optionsSuccessStatus: 204,
-    exposedHeaders: ["mcp-session-id", "last-event-id", "mcp-protocol-version"],
-  })
+  cors(
+    createCorsOptions({
+      methods: "GET,POST,DELETE",
+      exposedHeaders: [
+        "mcp-session-id",
+        "last-event-id",
+        "mcp-protocol-version",
+      ],
+    })
+  )
 );
 
 // Map sessionId to server transport for each client
@@ -198,9 +203,15 @@ app.delete("/mcp", async (req: Request, res: Response) => {
 });
 
 // Start the server
-const PORT = process.env.PORT || 3001;
-const server = app.listen(PORT, () => {
-  console.error(`MCP Streamable HTTP Server listening on port ${PORT}`);
+const PORT = process.env.PORT ? Number(process.env.PORT) : 3001;
+if (!Number.isFinite(PORT)) {
+  throw new Error(`Invalid PORT=${JSON.stringify(process.env.PORT)}`);
+}
+const HOST = process.env.HOST || "127.0.0.1";
+const server = app.listen(PORT, HOST, () => {
+  console.error(
+    `MCP Streamable HTTP Server listening on http://${HOST}:${PORT}`
+  );
 });
 
 // Handle server errors