Definition - What does Passive Attack mean?
A passive attack, in computing security, is an attack characterized by the attacker listening in on communication. In such an attack, the intruder/hacker does not attempt to break into the system or otherwise change data.
Techopedia explains Passive Attack
Passive attacks basically mean that the attacker is eavesdropping. This is in comparison to an active attack, where the intruder attemps to break into the system. Even though a passive attack sounds less harmful, the damage in the end can be just as severe if the right type of information is obtained.