Which of the following is a passive vulnerability assessment technique?

To determine which of the following is a passive vulnerability assessment technique, we need to understand what passive vulnerability assessment entails.

Passive vulnerability assessment techniques involve gathering information about a system or network without actively engaging with it. This means that the assessment is conducted in a non-intrusive manner, without directly interacting with the target system.

Now, let's examine the options provided and determine which one aligns with the definition of a passive vulnerability assessment technique:

1. Penetration testing: Penetration testing is an active vulnerability assessment technique that involves actively attempting to exploit vulnerabilities in a system or network. It is not a passive technique.

2. Network scanning: Network scanning can be both active and passive, depending on the approach used. Active network scanning involves sending packets to the target system to gather information, while passive network scanning involves monitoring network traffic to gather information. Since passive network scanning aligns with the definition of a passive vulnerability assessment technique, it could be the correct answer.

3. Social engineering: Social engineering is a technique that involves manipulating individuals to gain unauthorized access to systems or sensitive information. It is not a vulnerability assessment technique, whether passive or active.

4. Vulnerability scanning: Vulnerability scanning is an active vulnerability assessment technique that involves actively scanning a system or network for known vulnerabilities. It is not a passive technique.

Based on the analysis above, the option that aligns with the definition of a passive vulnerability assessment technique is passive network scanning.