Passive reconnaissance is a form of targeted information data collection that takes place when an individual’s personal data, such as a password, is stolen without the targeted individual's knowledge. Passive reconnaissance is defined as a malicious hack whereby the target may be observed, and they may be impersonated by the hacker for data collection purposes.

Passive reconnaissance can take place when the hacker is sifting through the targeted individual’s garbage in order to obtain discarded papers, which may contain the private information required to follow through with a malicious act online. Discarded computers are another source of information for hackers.

Traffic activity on a modem can be read and analyzed during passive reconnaissance by remotely analyzing the LED traffic activity lights. Secure shell (SSH) keystroke timing is another way that passive reconnaissance is enlisted.