A passive attack is a network attack in which a system is monitored and sometimes scanned for open ports and vulnerabilities. The purpose is solely to gain information about the target and no data is changed on the target.
Passive attacks include active reconnaissance and passive reconnaissance. In passive reconnaissance, an intruder monitors systems for vulnerabilities without interaction, through methods like session capture. In active reconnaissance, the intruder engages with the target system through methods like port scans.
Methods of passive attacks:
War driving detects vulnerable Wi-Fi networks by scanning them from nearby locations with a portable antenna. The attack is typically carried out from a moving vehicle, sometimes with GPS systems that hackers use to plot out areas with vulnerabilities on a map. War driving can be done just to steal an Internet connection or as a preliminary activity for a future attack.
In dumpster diving, intruders look for information stored on discarded computers and other devices or even passwords in trash bins. The intruders can then use this information to facilitate covert entry to a network or system.
An intruder might masquerade as an authorized network user and spy without interaction. With that access, an intruder might monitor network traffic by setting the network adapter to promiscuous mode.
A passive attack contrasts with an active attack, in which an intruder attempts to alter data on the target system or data en route for the target system.