what is passive attack

A passive attack on a cryptosystem is one in which the cryptanalyst cannot interact with any of the parties involved, attempting to break the system solely based upon observed data (i.e. the ciphertext). This can also include known plaintext attacks where both the plaintext and its corresponding ciphertext are known.While most classical ciphers are vulnerable to this form of attack, most modern ciphers are designed to prevent this type of attack above all others.

                                                                          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.