feat: improve web_fetch URL validation and security

Add URL validation and redirect limits to web_fetch tool to prevent potential security issues:

- Add _validate_url() function to validate URLs before fetching
  - Only allow http:// and https:// schemes (prevent file://, ftp://, etc.)
  - Verify URL has valid scheme and domain
  - Return descriptive error messages for invalid URLs

- Limit HTTP redirects to 5 (down from default 20) to prevent DoS attacks
  - Add MAX_REDIRECTS constant for easy configuration
  - Explicitly configure httpx.AsyncClient with max_redirects parameter

- Improve error handling with JSON error responses for validation failures

This addresses security concerns identified in code review where web_fetch
had no URL validation or redirect limits, potentially allowing:
- Unsafe URL schemes (file://, etc.)
- Redirect-based DoS attacks
- Invalid URL formats causing unclear errors

nanobot/agent/tools/web.py

@@ -5,6 +5,7 @@ import json
 import os
 import re
 from typing import Any
+from urllib.parse import urlparse
 
 import httpx
 
@@ -12,6 +13,7 @@ from nanobot.agent.tools.base import Tool
 
 # Shared constants
 USER_AGENT = "Mozilla/5.0 (Macintosh; Intel Mac OS X 14_7_2) AppleWebKit/537.36"
+MAX_REDIRECTS = 5  # Limit redirects to prevent DoS attacks
 
 
 def _strip_tags(text: str) -> str:
@@ -28,6 +30,33 @@ def _normalize(text: str) -> str:
     return re.sub(r'\n{3,}', '\n\n', text).strip()
 
 
+def _validate_url(url: str) -> tuple[bool, str]:
+    """
+    Validate URL for security.
+
+    Returns:
+        (is_valid, error_message): Tuple of validation result and error message if invalid.
+    """
+    try:
+        parsed = urlparse(url)
+
+        # Check if scheme exists
+        if not parsed.scheme:
+            return False, "URL must include a scheme (http:// or https://)"
+
+        # Only allow http and https schemes
+        if parsed.scheme.lower() not in ('http', 'https'):
+            return False, f"Invalid URL scheme '{parsed.scheme}'. Only http:// and https:// are allowed"
+
+        # Check if netloc (domain) exists
+        if not parsed.netloc:
+            return False, "URL must include a valid domain"
+
+        return True, ""
+    except Exception as e:
+        return False, f"Invalid URL format: {str(e)}"
+
+
 class WebSearchTool(Tool):
     """Search the web using Brave Search API."""
     
@@ -95,12 +124,21 @@ class WebFetchTool(Tool):
     
     async def execute(self, url: str, extractMode: str = "markdown", maxChars: int | None = None, **kwargs: Any) -> str:
         from readability import Document
-        
+
         max_chars = maxChars or self.max_chars
-        
+
+        # Validate URL before fetching
+        is_valid, error_msg = _validate_url(url)
+        if not is_valid:
+            return json.dumps({"error": f"URL validation failed: {error_msg}", "url": url})
+
         try:
-            async with httpx.AsyncClient() as client:
-                r = await client.get(url, headers={"User-Agent": USER_AGENT}, follow_redirects=True, timeout=30.0)
+            async with httpx.AsyncClient(
+                follow_redirects=True,
+                max_redirects=MAX_REDIRECTS,
+                timeout=30.0
+            ) as client:
+                r = await client.get(url, headers={"User-Agent": USER_AGENT})
                 r.raise_for_status()
             
             ctype = r.headers.get("content-type", "")