The Tenable Passive Vulnerability Scanner (PVS) is a most interesting product. It is truly passive in that it does not perform active scans of any kind. It is, simply, a very smart sniffer. The product depends for its usefulness on the way that it collects and reports vulnerability data. Since the PVS is always listening, it constantly collects information from the normal data flows on the network. This is superior to active scanners in two important ways.