fix: Bind client to localhost by default to prevent DNS rebinding attacks

Complete the security hardening started in e8e9909 by also binding the client to localhost only.
Previously only the server was protected while the client remained exposed to the network,
allowing attackers to access the server through the client as a proxy.

Changes:
- Add HOST environment variable support to client (prod mode)
- Configure Vite dev server to bind to localhost by default
- Update browser auto-open URLs to use actual host instead of hardcoded 127.0.0.1
- Fix missing cancelled parameter in startProdClient function

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: felixweinberger

README.md

@@ -168,13 +168,13 @@ DANGEROUSLY_OMIT_AUTH=true npm start
 
 #### Local-only Binding
 
-By default, the MCP Inspector proxy server binds only to `127.0.0.1` (localhost) to prevent network access. This ensures the server is not accessible from other devices on the network. If you need to bind to all interfaces for development purposes, you can override this with the `HOST` environment variable:
+By default, both the MCP Inspector proxy server and client bind only to `127.0.0.1` (localhost) to prevent network access. This ensures they are not accessible from other devices on the network. If you need to bind to all interfaces for development purposes, you can override this with the `HOST` environment variable:
 
 ```bash
 HOST=0.0.0.0 npm start
 ```
 
-**Warning:** Only bind to all interfaces in trusted network environments, as this exposes the proxy server's ability to execute local processes.
+**Warning:** Only bind to all interfaces in trusted network environments, as this exposes the proxy server's ability to execute local processes and both services to network access.
 
 #### DNS Rebinding Protection
 

client/bin/client.js

@@ -40,18 +40,19 @@ const server = http.createServer((request, response) => {
 });
 
 const port = process.env.PORT || 6274;
+const host = process.env.HOST || "127.0.0.1";
 server.on("listening", () => {
   console.log(
-    `🔍 MCP Inspector is up and running at http://127.0.0.1:${port} 🚀`,
+    `🔍 MCP Inspector is up and running at http://${host}:${port} 🚀`,
   );
 });
 server.on("error", (err) => {
   if (err.message.includes(`EADDRINUSE`)) {
     console.error(
-      `❌  MCP Inspector PORT IS IN USE at http://127.0.0.1:${port} ❌ `,
+      `❌  MCP Inspector PORT IS IN USE at http://${host}:${port} ❌ `,
     );
   } else {
     throw err;
   }
 });
-server.listen(port);
+server.listen(port, host);

client/bin/start.js

@@ -102,7 +102,8 @@ async function startDevClient(clientOptions) {
   const { CLIENT_PORT, authDisabled, sessionToken, abort, cancelled } =
     clientOptions;
   const clientCommand = "npx";
-  const clientArgs = ["vite", "--port", CLIENT_PORT];
+  const host = process.env.HOST || "127.0.0.1";
+  const clientArgs = ["vite", "--port", CLIENT_PORT, "--host", host];
 
   const client = spawn(clientCommand, clientArgs, {
     cwd: resolve(__dirname, ".."),
@@ -113,9 +114,10 @@ async function startDevClient(clientOptions) {
 
   // Auto-open browser after vite starts
   if (process.env.MCP_AUTO_OPEN_ENABLED !== "false") {
+    const clientHost = process.env.HOST || "127.0.0.1";
     const url = authDisabled
-      ? `http://127.0.0.1:${CLIENT_PORT}`
-      : `http://127.0.0.1:${CLIENT_PORT}/?MCP_PROXY_AUTH_TOKEN=${sessionToken}`;
+      ? `http://${clientHost}:${CLIENT_PORT}`
+      : `http://${clientHost}:${CLIENT_PORT}/?MCP_PROXY_AUTH_TOKEN=${sessionToken}`;
 
     // Give vite time to start before opening browser
     setTimeout(() => {
@@ -139,7 +141,8 @@ async function startDevClient(clientOptions) {
 }
 
 async function startProdClient(clientOptions) {
-  const { CLIENT_PORT, authDisabled, sessionToken, abort } = clientOptions;
+  const { CLIENT_PORT, authDisabled, sessionToken, abort, cancelled } =
+    clientOptions;
   const inspectorClientPath = resolve(
     __dirname,
     "../..",
@@ -148,11 +151,12 @@ async function startProdClient(clientOptions) {
     "client.js",
   );
 
-  // Auto-open browser with token
-  if (process.env.MCP_AUTO_OPEN_ENABLED !== "false") {
+  // Only auto-open browser if not cancelled
+  if (process.env.MCP_AUTO_OPEN_ENABLED !== "false" && !cancelled) {
+    const clientHost = process.env.HOST || "127.0.0.1";
     const url = authDisabled
-      ? `http://127.0.0.1:${CLIENT_PORT}`
-      : `http://127.0.0.1:${CLIENT_PORT}/?MCP_PROXY_AUTH_TOKEN=${sessionToken}`;
+      ? `http://${clientHost}:${CLIENT_PORT}`
+      : `http://${clientHost}:${CLIENT_PORT}/?MCP_PROXY_AUTH_TOKEN=${sessionToken}`;
     open(url);
   }