Probabilistic tracking of host characteristics

A system for mitigating network attacks includes a protected network and one or more attack mitigation devices communicatively coupled to the protected network. The attack mitigation devices are configured to receive a request from a host having an IP address and determine whether the IP address is included in a first probabilistic data structure representing addresses of hosts having failed to authenticate using a first authentication procedure. The attack mitigation devices are also configured to perform the first authentication procedure, responsive to a determination that the IP address of the host is not included in the first data structure. The attack mitigation devices are yet further configured to allow the host to access the protected network, responsive to successful completion of the first authentication procedure and to update the first data structure to include the IP address of the host, responsive to unsuccessful completion of the first authentication procedure.