fix: path traversal vulnerability - use prefix matching in is_dangerous_path()

The is_dangerous_path() function only did exact string matching,
allowing attackers to bypass protection by accessing subdirectories:
- /etc was blocked but /etc/passwd was allowed
- C:\Windows was blocked but C:\Windows\System32\... was allowed

This minimal fix changes is_dangerous_path() to use PREFIX MATCHING:
- Now blocks dangerous directories AND all their subdirectories
- Paths like /etcbackup are still allowed (not under /etc)
- No changes to DANGEROUS_PATHS list

Security:
- Fixes CWE-22: Path Traversal vulnerability
- Reported by: Team off-course (K-Shield.Jr 15th)

Fixes #312
Fixes #293

utils/security_config.py

@@ -89,16 +89,47 @@ EXCLUDED_DIRS = {
 
 def is_dangerous_path(path: Path) -> bool:
     """
-    Check if a path is in the dangerous paths list.
+    Check if a path is in or under a dangerous directory.
+
+    Uses PREFIX MATCHING to block dangerous directories AND their subdirectories.
+    For example, if "/etc" is in DANGEROUS_PATHS, both "/etc" and "/etc/passwd"
+    will be blocked.
 
     Args:
         path: Path to check
 
     Returns:
         True if the path is dangerous and should not be accessed
+
+    Security:
+        Fixes path traversal vulnerability (CWE-22) reported in:
+        - https://github.com/BeehiveInnovations/zen-mcp-server/issues/293
+        - https://github.com/BeehiveInnovations/zen-mcp-server/issues/312
     """
     try:
         resolved = path.resolve()
-        return str(resolved) in DANGEROUS_PATHS or resolved.parent == resolved
+        resolved_str = str(resolved)
+
+        # Check 1: Root directory (filesystem root)
+        if resolved.parent == resolved:
+            return True
+
+        # Check 2: Exact match or subdirectory of dangerous paths
+        for dangerous in DANGEROUS_PATHS:
+            # Skip root "/" - already handled above
+            if dangerous == "/":
+                continue
+
+            # Exact match
+            if resolved_str == dangerous:
+                return True
+
+            # Subdirectory check: path starts with dangerous + separator
+            # Use os.sep for platform-appropriate separator
+            if resolved_str.startswith(dangerous + "/") or resolved_str.startswith(dangerous + "\\"):
+                return True
+
+        return False
+
     except Exception:
         return True  # If we can't resolve, consider it dangerous