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

tests/test_path_traversal_security.py

@@ -0,0 +1,65 @@
+"""
+Test path traversal security fix.
+
+Fixes vulnerability reported in:
+- https://github.com/BeehiveInnovations/zen-mcp-server/issues/293
+- https://github.com/BeehiveInnovations/zen-mcp-server/issues/312
+
+The vulnerability: is_dangerous_path() only did exact string matching,
+so /etc was blocked but /etc/passwd was allowed.
+"""
+
+from pathlib import Path
+
+from utils.security_config import is_dangerous_path
+
+
+class TestPathTraversalFix:
+    """Test that subdirectories of dangerous paths are now blocked."""
+
+    def test_exact_match_still_works(self):
+        """Test that exact dangerous paths are still blocked."""
+        assert is_dangerous_path(Path("/etc")) is True
+        assert is_dangerous_path(Path("/usr")) is True
+        assert is_dangerous_path(Path("/var")) is True
+
+    def test_subdirectory_now_blocked(self):
+        """Test that subdirectories are now blocked (the fix)."""
+        # These were allowed before the fix
+        assert is_dangerous_path(Path("/etc/passwd")) is True
+        assert is_dangerous_path(Path("/etc/shadow")) is True
+        assert is_dangerous_path(Path("/etc/hosts")) is True
+        assert is_dangerous_path(Path("/var/log/auth.log")) is True
+
+    def test_deeply_nested_blocked(self):
+        """Test that deeply nested paths are blocked."""
+        assert is_dangerous_path(Path("/etc/ssh/sshd_config")) is True
+        assert is_dangerous_path(Path("/usr/local/bin/python")) is True
+
+    def test_root_blocked(self):
+        """Test that root directory is blocked."""
+        assert is_dangerous_path(Path("/")) is True
+
+    def test_safe_paths_allowed(self):
+        """Test that safe paths are still allowed."""
+        # User project directories should be allowed
+        assert is_dangerous_path(Path("/tmp/test")) is False
+        assert is_dangerous_path(Path("/tmp/myproject/src")) is False
+
+    def test_similar_names_not_blocked(self):
+        """Test that paths with similar names are not blocked."""
+        # /etcbackup should NOT be blocked (it's not under /etc)
+        assert is_dangerous_path(Path("/tmp/etcbackup")) is False
+        assert is_dangerous_path(Path("/tmp/my_etc_files")) is False
+
+
+class TestRegressionPrevention:
+    """Regression tests for the specific vulnerability."""
+
+    def test_etc_passwd_blocked(self):
+        """Test /etc/passwd is blocked (common attack target)."""
+        assert is_dangerous_path(Path("/etc/passwd")) is True
+
+    def test_etc_shadow_blocked(self):
+        """Test /etc/shadow is blocked (password hashes)."""
+        assert is_dangerous_path(Path("/etc/shadow")) is True