Comment by tialaramex

The exact technologies used for "access cards" have varied over the years, but last time I checked most of them still aren't doing anything even vaguely secure. The card says "I'm card 1234-5678" and the access system checks that is on the list, welcome in. Like magnetic stripes it isn't obvious to the human operator, but just like magnetic stripes you can just clone it by listening and reciting the same, "I'm card 1234-5678".

Now to be fair, you'd often find these systems are so clumsily installed that you don't need to clone a card anyway, the out-of-hours access has an "emergency" generic key you can buy from a hardware store, the controller was placed on the wrong side of the door - that sort of thing. But even a well-installed system is typically vulnerable to a competent intruder.