Which of the following is NOT a common type of network security threat?

The correct choice identifies firewall breaches as not being a common type of network security threat in the same way that the others are categorized.

Phishing, malware, and ransomware represent direct threats or attacks targeting individuals or systems. Phishing is a method used to trick users into revealing sensitive information by impersonating legitimate entities. Malware encompasses a wide range of malicious software designed to harm or exploit devices, including viruses and Trojans. Ransomware specifically targets data by encrypting it and demanding payment for its release, directly impacting the victim's access to their information.

On the other hand, firewall breaches refer to a failure or bypassing of the firewall's protective measures rather than an active threat in the same manner. Firewalls are security systems designed to monitor and control incoming and outgoing network traffic based on predetermined security rules. A breach indicates a vulnerability or flaw in the existing security measures rather than a tactical attack, which is what the other options represent. Therefore, while it is crucial to be aware of firewall vulnerabilities, they do not constitute a type of threat in the conventional sense applicable to Phishing, Malware, or Ransomware.