What is a common sign of a successful phishing attack?

A common sign of a successful phishing attack is unusual account activity. When a phishing attack is successful, it often leads to unauthorized access to an individual's account or sensitive information. This can manifest in various ways, such as unexpected transactions, changes to account settings, the presence of unfamiliar login locations, or messages indicating account access from unknown devices. Monitoring for such unusual activities is critical for identifying and responding to potential security breaches that may result from phishing schemes.

The other options do not directly indicate a successful phishing attack. Strong passwords reflect good security practices and can help prevent unauthorized access, while accepted security protocols and regular system updates are measures designed to enhance overall cybersecurity and reduce the risk of attacks, rather than direct indicators of an attack's success.