fix: Add WebSocket security - authentication, rate limiting, localhost binding (#1504)

This commit addresses the security vulnerability where the WebSocket
gateway was exposed on 0.0.0.0 without authentication.

## Changes

### Security Improvements
1. **Localhost binding by default**: Changed HOST from "0.0.0.0" to "127.0.0.1"
   - Gateway now only listens on localhost by default
   - External binding possible via NEXUS_WS_HOST environment variable

2. **Token-based authentication**: Added NEXUS_WS_TOKEN environment variable
   - If set, clients must send auth message with valid token
   - If not set, no authentication required (backward compatible)
   - Auth timeout: 5 seconds

3. **Rate limiting**:
   - Connection rate limiting: 10 connections per IP per 60 seconds
   - Message rate limiting: 100 messages per connection per 60 seconds
   - Configurable via constants

4. **Enhanced logging**:
   - Logs security configuration on startup
   - Warns if authentication is disabled
   - Warns if binding to 0.0.0.0

### Configuration
Environment variables:
- NEXUS_WS_HOST: Host to bind to (default: 127.0.0.1)
- NEXUS_WS_PORT: Port to listen on (default: 8765)
- NEXUS_WS_TOKEN: Authentication token (empty = no auth)

### Backward Compatibility
- Default behavior is now secure (localhost only)
- No authentication by default (same as before)
- Existing clients will work without changes
- External binding possible via NEXUS_WS_HOST=0.0.0.0

## Security Impact
- Prevents unauthorized access from external networks
- Prevents connection flooding
- Prevents message flooding
- Maintains backward compatibility

Fixes #1504

server.py

@@ -3,20 +3,34 @@
 The Nexus WebSocket Gateway — Robust broadcast bridge for Timmy's consciousness.
 This server acts as the central hub for the-nexus, connecting the mind (nexus_think.py),
 the body (Evennia/Morrowind), and the visualization surface.
+
+Security features:
+- Binds to 127.0.0.1 by default (localhost only)
+- Optional external binding via NEXUS_WS_HOST environment variable
+- Token-based authentication via NEXUS_WS_TOKEN environment variable
+- Rate limiting on connections
+- Connection logging and monitoring
 """
 import asyncio
 import json
 import logging
+import os
 import signal
 import sys
-from typing import Set
+import time
+from typing import Set, Dict, Optional
+from collections import defaultdict
 
 # Branch protected file - see POLICY.md
 import websockets
 
 # Configuration
-PORT = 8765
-HOST = "0.0.0.0"  # Allow external connections if needed
+PORT = int(os.environ.get("NEXUS_WS_PORT", "8765"))
+HOST = os.environ.get("NEXUS_WS_HOST", "127.0.0.1")  # Default to localhost only
+AUTH_TOKEN = os.environ.get("NEXUS_WS_TOKEN", "")  # Empty = no auth required
+RATE_LIMIT_WINDOW = 60  # seconds
+RATE_LIMIT_MAX_CONNECTIONS = 10  # max connections per IP per window
+RATE_LIMIT_MAX_MESSAGES = 100  # max messages per connection per window
 
 # Logging setup
 logging.basicConfig(
@@ -28,15 +42,97 @@ logger = logging.getLogger("nexus-gateway")
 
 # State
 clients: Set[websockets.WebSocketServerProtocol] = set()
+connection_tracker: Dict[str, list] = defaultdict(list)  # IP -> [timestamps]
+message_tracker: Dict[int, list] = defaultdict(list)  # connection_id -> [timestamps]
+
+def check_rate_limit(ip: str) -> bool:
+    """Check if IP has exceeded connection rate limit."""
+    now = time.time()
+    # Clean old entries
+    connection_tracker[ip] = [t for t in connection_tracker[ip] if now - t < RATE_LIMIT_WINDOW]
+    
+    if len(connection_tracker[ip]) >= RATE_LIMIT_MAX_CONNECTIONS:
+        return False
+    
+    connection_tracker[ip].append(now)
+    return True
+
+def check_message_rate_limit(connection_id: int) -> bool:
+    """Check if connection has exceeded message rate limit."""
+    now = time.time()
+    # Clean old entries
+    message_tracker[connection_id] = [t for t in message_tracker[connection_id] if now - t < RATE_LIMIT_WINDOW]
+    
+    if len(message_tracker[connection_id]) >= RATE_LIMIT_MAX_MESSAGES:
+        return False
+    
+    message_tracker[connection_id].append(now)
+    return True
+
+async def authenticate_connection(websocket: websockets.WebSocketServerProtocol) -> bool:
+    """Authenticate WebSocket connection using token."""
+    if not AUTH_TOKEN:
+        # No authentication required
+        return True
+    
+    try:
+        # Wait for authentication message (first message should be auth)
+        auth_message = await asyncio.wait_for(websocket.recv(), timeout=5.0)
+        auth_data = json.loads(auth_message)
+        
+        if auth_data.get("type") != "auth":
+            logger.warning(f"Invalid auth message type from {websocket.remote_address}")
+            return False
+        
+        token = auth_data.get("token", "")
+        if token != AUTH_TOKEN:
+            logger.warning(f"Invalid auth token from {websocket.remote_address}")
+            return False
+        
+        logger.info(f"Authenticated connection from {websocket.remote_address}")
+        return True
+        
+    except asyncio.TimeoutError:
+        logger.warning(f"Authentication timeout from {websocket.remote_address}")
+        return False
+    except json.JSONDecodeError:
+        logger.warning(f"Invalid auth JSON from {websocket.remote_address}")
+        return False
+    except Exception as e:
+        logger.error(f"Authentication error from {websocket.remote_address}: {e}")
+        return False
 
 async def broadcast_handler(websocket: websockets.WebSocketServerProtocol):
     """Handles individual client connections and message broadcasting."""
-    clients.add(websocket)
     addr = websocket.remote_address
+    ip = addr[0] if addr else "unknown"
+    connection_id = id(websocket)
+    
+    # Check connection rate limit
+    if not check_rate_limit(ip):
+        logger.warning(f"Connection rate limit exceeded for {ip}")
+        await websocket.close(1008, "Rate limit exceeded")
+        return
+    
+    # Authenticate if token is required
+    if not await authenticate_connection(websocket):
+        await websocket.close(1008, "Authentication failed")
+        return
+    
+    clients.add(websocket)
     logger.info(f"Client connected from {addr}. Total clients: {len(clients)}")
     
     try:
         async for message in websocket:
+            # Check message rate limit
+            if not check_message_rate_limit(connection_id):
+                logger.warning(f"Message rate limit exceeded for {addr}")
+                await websocket.send(json.dumps({
+                    "type": "error",
+                    "message": "Message rate limit exceeded"
+                }))
+                continue
+            
             # Parse for logging/validation if it's JSON
             try:
                 data = json.loads(message)
@@ -81,6 +177,20 @@ async def broadcast_handler(websocket: websockets.WebSocketServerProtocol):
 
 async def main():
     """Main server loop with graceful shutdown."""
+    # Log security configuration
+    if AUTH_TOKEN:
+        logger.info("Authentication: ENABLED (token required)")
+    else:
+        logger.warning("Authentication: DISABLED (no token required)")
+    
+    if HOST == "0.0.0.0":
+        logger.warning("Host binding: 0.0.0.0 (all interfaces) - SECURITY RISK")
+    else:
+        logger.info(f"Host binding: {HOST} (localhost only)")
+    
+    logger.info(f"Rate limiting: {RATE_LIMIT_MAX_CONNECTIONS} connections/IP/{RATE_LIMIT_WINDOW}s, "
+                f"{RATE_LIMIT_MAX_MESSAGES} messages/connection/{RATE_LIMIT_WINDOW}s")
+    
     logger.info(f"Starting Nexus WS gateway on ws://{HOST}:{PORT}")
     
     # Set up signal handlers for graceful shutdown