default field permissions
I found the answer. It is done using the hook_field_access. In my case, what I did was:

function my_module_field_access($op, $field, $type, $entity) {
    if ($op=="edit" &amp;&amp; $field["field_name"]=="my_field") {
        if (!user_access("administer users"))
            return FALSE;
    }
}