In the scenario described, where the confidentiality and integrity of financial data must take precedence over system availability, the financial institution should configure the network security appliance to use a "fail-secure" mode.
Fail-secure means that in the event of a failure, the system will take actions that prioritize the protection of sensitive data. This could involve shutting down access to the network or the data rather than allowing potentially insecure or compromised access. By implementing a fail-secure configuration, the bank ensures that the financial data remains protected, as any potential breach or vulnerability would not be exploited while the system is in a failed state.
In summary, the appropriate failure mode configuration for the appliance should be set to fail-secure.