The vulnerability let an attacker crash a computer running Microsoft's WebTV software simply by sending it a certain type of information over the Internet. The problem affected only computers on which the WebTV software had been installed and was running.
The WebTV software, not to be confused with the special-purpose hardware that goes by the same name, lets a person watch television on a computer.
Bug hunter Andrew Griffiths notified Microsoft of the attack in August and publicly announced it in September, but Microsoft refused to acknowledge whether it was in fact a problem until Wednesday. Microsoft now has come around to Griffiths' point of view.