Which of these exploits would an incident responder most likely consider to be a passive online attack?

In the context of cybersecurity, a passive online attack occurs when an attacker monitors or captures data without actively interfering with the systems or the data flow. Passive attacks typically include activities such as eavesdropping or sniffing network traffic to gather sensitive information.

The activity of sniffing Ethernet traffic with a tool like Wireshark to intercept cleartext passwords falls squarely into this category. In this instance, the attacker is observing network packets and capturing potentially sensitive data like passwords as they are transmitted over the network. This does not alter the network traffic or disrupt any services; rather, it simply involves monitoring existing communications.

The other options represent active attacks or methods that involve some form of intervention or manipulation in the system or traffic. For instance, SQL injection directly compromises the database by inserting malicious queries, a Denial of Service attack disrupts service availability, and Cross-Site Scripting targets users by injecting malicious scripts into web pages viewed by others. All of these activities involve some level of interaction with the system or its users, thereby categorizing them as active attacks rather than passive ones.