The IS auditor's next course of action after finding that firewalls are outdated and not supported by vendors should be to determine the risk of not replacing the firewall. Outdated firewalls may have known vulnerabilities that can be exploited by attackers to bypass security controls and access the network. They may also lack compatibility with newer technologies or standards that are required for optimal network performance and protection. Not replacing the firewall could expose the organization to various threats, such as data breaches, denial-of-service attacks, malware infections, or regulatory non-compliance. The IS auditor should assess the likelihood and impact of these threats and quantify the risk level for management to make informed decisions.