diff --git a/.python-version b/.python-version
index 24ee5b1b..3a4f41ef 100644
--- a/.python-version
+++ b/.python-version
@@ -1 +1 @@
-3.13
+3.13
\ No newline at end of file
diff --git a/fix_env.py b/fix_env.py
index 50dc9a5f..4f975cb6 100644
--- a/fix_env.py
+++ b/fix_env.py
@@ -1,5 +1,6 @@
 import os
 import re
+import stat
 
 def fix_env():
     try:
@@ -61,7 +62,12 @@ def escape_val(val):
     with open('.env', 'w') as f:
         f.write(new_content)
     
+    # SECURITY: Ensure .env is only readable by the owner (600) on Unix-like systems
+    if os.name != 'nt':
+        os.chmod('.env', stat.S_IRUSR | stat.S_IWUSR)
+
     print("Fixed .env file: standardized quotes and corrected variable assignments.")
+    print("Security: .env permissions set to 600 (read/write only by owner).")
 
 if __name__ == "__main__":
     fix_env()
diff --git a/main.py b/main.py
index 9f324efe..5a493e65 100644
--- a/main.py
+++ b/main.py
@@ -16,6 +16,7 @@
 import argparse
 import json
 import os
+import stat
 import logging
 import sys
 import time
@@ -92,6 +93,44 @@ def format(self, record):
 handler.setFormatter(ColoredFormatter())
 logging.basicConfig(level=logging.INFO, handlers=[handler])
 logging.getLogger("httpx").setLevel(logging.WARNING)
+
+
+def check_env_permissions(env_path: str = ".env") -> None:
+    """
+    Check .env file permissions and warn if readable by others.
+
+    Args:
+        env_path: Path to the .env file to check (default: ".env")
+    """
+    if not os.path.exists(env_path):
+        return
+
+    try:
+        file_stat = os.stat(env_path)
+        # Check if group or others have any permission
+        if file_stat.st_mode & (stat.S_IRWXG | stat.S_IRWXO):
+            platform_hint = (
+                "Please secure your .env file so it is only readable by "
+                "the owner."
+            )
+            if os.name != "nt":
+                platform_hint += " For example: 'chmod 600 .env'."
+            perms = format(stat.S_IMODE(file_stat.st_mode), '03o')
+            sys.stderr.write(
+                f"{Colors.WARNING}⚠️  Security Warning: .env file is "
+                f"readable by others ({perms})! {platform_hint}"
+                f"{Colors.ENDC}\n"
+            )
+    except Exception as error:
+        exception_type = type(error).__name__
+        sys.stderr.write(
+            f"{Colors.WARNING}⚠️  Security Warning: Could not check .env "
+            f"permissions ({exception_type}: {error}){Colors.ENDC}\n"
+        )
+
+
+# SECURITY: Check .env permissions (after Colors is defined for NO_COLOR support)
+check_env_permissions()
 log = logging.getLogger("control-d-sync")
 
 # --------------------------------------------------------------------------- #
diff --git a/tests/test_security.py b/tests/test_security.py
new file mode 100644
index 00000000..b82f2c2c
--- /dev/null
+++ b/tests/test_security.py
@@ -0,0 +1,156 @@
+"""
+Tests for security features in main.py and fix_env.py
+"""
+import os
+import stat
+import sys
+from unittest.mock import MagicMock
+import pytest
+import fix_env
+
+
+@pytest.mark.skipif(os.name == 'nt', reason="Unix permissions not applicable on Windows")
+def test_env_permission_check_warns_on_insecure_permissions(monkeypatch, tmp_path):
+    """Test that insecure .env permissions trigger a warning."""
+    # Import main to get access to check_env_permissions and Colors
+    import main
+
+    # Create a temporary .env file
+    env_file = tmp_path / ".env"
+    env_file.write_text("TOKEN=test")
+    os.chmod(env_file, 0o644)
+
+    # Mock sys.stderr to capture warnings
+    mock_stderr = MagicMock()
+    monkeypatch.setattr(sys, "stderr", mock_stderr)
+
+    # Run the permission check logic
+    main.check_env_permissions(str(env_file))
+
+    # Verify warning was written
+    mock_stderr.write.assert_called()
+    call_args = mock_stderr.write.call_args[0][0]
+    assert "Security Warning" in call_args
+    assert "readable by others" in call_args
+    assert "644" in call_args
+
+
+@pytest.mark.skipif(os.name == 'nt', reason="Unix permissions not applicable on Windows")
+def test_env_permission_check_no_warn_on_secure_permissions(monkeypatch, tmp_path):
+    """Test that secure .env permissions do not trigger a warning."""
+    # Import main to get access to check_env_permissions
+    import main
+
+    # Create a temporary .env file
+    env_file = tmp_path / ".env"
+    env_file.write_text("TOKEN=test")
+    os.chmod(env_file, 0o600)
+
+    # Mock sys.stderr to capture warnings
+    mock_stderr = MagicMock()
+    monkeypatch.setattr(sys, "stderr", mock_stderr)
+
+    # Run the permission check logic
+    main.check_env_permissions(str(env_file))
+
+    # Verify no warning was written
+    mock_stderr.write.assert_not_called()
+
+
+def test_env_permission_check_handles_stat_error(monkeypatch):
+    """Test that permission check handles stat() errors gracefully."""
+    # Import main to get access to check_env_permissions
+    import main
+
+    # Mock sys.stderr to capture warnings
+    mock_stderr = MagicMock()
+    monkeypatch.setattr(sys, "stderr", mock_stderr)
+
+    # Mock os.stat to raise an exception
+    def mock_stat(path):
+        raise PermissionError("Cannot access file")
+
+    monkeypatch.setattr(os, "stat", mock_stat)
+    # Mock os.path.exists to return True so the check proceeds
+    monkeypatch.setattr(os.path, "exists", lambda x: True)
+
+    # Run the permission check - should handle the error gracefully
+    main.check_env_permissions(".env")
+
+    # Verify error warning was written
+    mock_stderr.write.assert_called()
+    call_args = mock_stderr.write.call_args[0][0]
+    assert "Security Warning" in call_args
+    assert "Could not check .env permissions" in call_args
+    assert "PermissionError" in call_args
+
+
+@pytest.mark.skipif(os.name == 'nt', reason="Unix permissions not applicable on Windows")
+def test_fix_env_sets_secure_permissions(tmp_path, monkeypatch):
+    """Test that fix_env.py sets secure permissions on .env file."""
+    # Change to temp directory
+    monkeypatch.chdir(tmp_path)
+
+    # Create a .env file with insecure content
+    env_file = tmp_path / ".env"
+    env_file.write_text("TOKEN='test_token'\nPROFILE='test_profile'\n")
+
+    # Set insecure permissions initially
+    os.chmod(env_file, 0o644)
+
+    # Run fix_env
+    fix_env.fix_env()
+
+    # Check that permissions are now secure
+    file_stat = os.stat(env_file)
+    mode = stat.S_IMODE(file_stat.st_mode)
+
+    # Verify permissions are 600 (read/write for owner only)
+    assert mode == 0o600, f"Expected 600, got {oct(mode)}"
+
+    # Verify no group or other permissions
+    assert not (file_stat.st_mode & stat.S_IRWXG), "Group has permissions"
+    assert not (file_stat.st_mode & stat.S_IRWXO), "Others have permissions"
+
+
+def test_fix_env_skips_chmod_on_windows(tmp_path, monkeypatch):
+    """Test that fix_env.py skips chmod on Windows."""
+    # Change to temp directory
+    monkeypatch.chdir(tmp_path)
+
+    # Create a .env file
+    env_file = tmp_path / ".env"
+    env_file.write_text("TOKEN='test_token'\nPROFILE='test_profile'\n")
+
+    # Mock os.name to simulate Windows
+    monkeypatch.setattr(os, "name", "nt")
+
+    # Mock os.chmod to verify it's not called
+    mock_chmod = MagicMock()
+    monkeypatch.setattr(os, "chmod", mock_chmod)
+
+    # Run fix_env
+    fix_env.fix_env()
+
+    # Verify chmod was not called on Windows
+    mock_chmod.assert_not_called()
+
+
+def test_octal_permission_format():
+    """Test that octal permission formatting is robust."""
+    # Test various permission modes
+    test_modes = [
+        0o644,  # rw-r--r--
+        0o600,  # rw-------
+        0o755,  # rwxr-xr-x
+        0o000,  # ---------
+    ]
+
+    for mode in test_modes:
+        # The robust way
+        result = format(stat.S_IMODE(mode), '03o')
+        # Should always be 3 digits
+        assert len(result) == 3, f"Expected 3 digits for {oct(mode)}, got {result}"
+        # Should be numeric
+        assert result.isdigit(), f"Expected numeric for {oct(mode)}, got {result}"
+