security: safer CORS + avoid shell exec in examples

Author: Theodor N. Engøy

examples/servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless/server.py

@@ -127,7 +127,9 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
     # for browser-based clients (ensures 500 errors get proper CORS headers)
     starlette_app = CORSMiddleware(
         starlette_app,
-        allow_origins=["*"],  # Allow all origins - adjust as needed for production
+        # For local/dev browser clients, allow localhost origins. For production, use
+        # a strict allowlist of known UI origins.
+        allow_origin_regex=r"https?://(localhost|127\\.0\\.0\\.1)(:\\d+)?",
         allow_methods=["GET", "POST", "DELETE"],  # MCP streamable HTTP methods
         expose_headers=["Mcp-Session-Id"],
     )

examples/servers/simple-streamablehttp/mcp_simple_streamablehttp/server.py

@@ -152,7 +152,9 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
     # for browser-based clients (ensures 500 errors get proper CORS headers)
     starlette_app = CORSMiddleware(
         starlette_app,
-        allow_origins=["*"],  # Allow all origins - adjust as needed for production
+        # For local/dev browser clients, allow localhost origins. For production, use
+        # a strict allowlist of known UI origins.
+        allow_origin_regex=r"https?://(localhost|127\\.0\\.0\\.1)(:\\d+)?",
         allow_methods=["GET", "POST", "DELETE"],  # MCP streamable HTTP methods
         expose_headers=["Mcp-Session-Id"],
     )

examples/snippets/clients/url_elicitation_client.py

@@ -24,8 +24,6 @@
 
 import asyncio
 import json
-import subprocess
-import sys
 import webbrowser
 from typing import Any
 from urllib.parse import urlparse
@@ -124,12 +122,9 @@ def extract_domain(url: str) -> str:
 def open_browser(url: str) -> None:
     """Open URL in the default browser."""
     try:
-        if sys.platform == "darwin":
-            subprocess.run(["open", url], check=False)
-        elif sys.platform == "win32":
-            subprocess.run(["start", url], shell=True, check=False)
-        else:
-            webbrowser.open(url)
+        if not webbrowser.open(url, new=2):
+            print("Could not open browser automatically.")
+            print(f"Please manually open: {url}")
     except Exception as e:
         print(f"Failed to open browser: {e}")
         print(f"Please manually open: {url}")

src/mcp/cli/cli.py

@@ -3,6 +3,7 @@
 import importlib.metadata
 import importlib.util
 import os
+import shutil
 import subprocess
 import sys
 from pathlib import Path
@@ -44,13 +45,10 @@ def _get_npx_command():
     if sys.platform == "win32":
         # Try both npx.cmd and npx.exe on Windows
         for cmd in ["npx.cmd", "npx.exe", "npx"]:
-            try:
-                subprocess.run([cmd, "--version"], check=True, capture_output=True, shell=True)
+            if shutil.which(cmd):
                 return cmd
-            except subprocess.CalledProcessError:
-                continue
         return None
-    return "npx"  # On Unix-like systems, just use npx
+    return shutil.which("npx")
 
 
 def _parse_env_var(env_var: str) -> tuple[str, str]:  # pragma: no cover