An exemplary method includes an attack traffic mitigation system 1) identifying a range of ports left open by a firewall for a network element to receive network traffic provided by a computing device, 2) designating a subset of one or more ports included in the range of open ports as being included in a legitimate port range configured to receive legitimate network traffic provided by the computing device, and 3) directing the network element to drop network traffic provided by the computing device and received by each port included in the range of open ports that is not included in the legitimate port range. Corresponding methods and systems are also disclosed.