The instant disclosure is directed to an attack/unwanted activity detecting firewall for use in protecting authentication-based network resources. The instant system is adapted for installation inline or in sniffer mode. In various embodiments, defined rules are applied to network traffic to determine whether certain types of attacks are occurring on the network resources. If one such attack is detected, the system provides for several potential responses, including for example disconnecting the attacking remote machine, requiring the user at that machine to re-authenticate, and/or requiring a second factor of authentication from the user at that machine. In some example embodiments, regardless of any activity required of a user at the remote machine suspected of malicious behavior, the disclosed system generates an alarm or other alert for presentation as appropriate, such as via a graphical user interface or a third-party system using an API.