What is the fail closed behavior of a firewall?

The fail closed behavior of a firewall refers to its approach to handling traffic when there is a failure or a system problem. In this context, when a firewall is configured to fail closed, it means that in the event of a failure—such as hardware issues, software malfunctions, or network disruptions—the firewall will block all traffic rather than allowing it to pass through. This is a security-oriented decision designed to prevent unauthorized access and protect sensitive resources during uncertain or potentially risky conditions.

By adopting this approach, a firewall ensures that if it cannot operate normally and fully enforce its rules, it will err on the side of caution by denying access to all traffic. This maximizes security and minimizes potential vulnerabilities that could be exploited in situations where the firewall's functionality is compromised.