  . As Supervisory Control and Data Acquisition (SCADA) systems control several
critical infrastructures, they have connected to the internet. Consequently,
SCADA systems face different sophisticated types of cyber adversaries. This
paper suggests a Probability Risk Identification based Intrusion Detection
System (PRI-IDS) technique based on analysing network traffic of Modbus TCP/IP
for identifying replay attacks. It is acknowledged that Modbus TCP is usually
vulnerable due to its unauthenticated and unencrypted nature. Our technique is
evaluated using a simulation environment by configuring a testbed, which is a
cus- tom SCADA network that is cheap, accurate and scalable. The testbed is
exploited when testing the IDS by sending individual packets from an attacker
located on the same LAN as the Modbus master and slave. The experimental
results demonstrated that the proposed technique can effectively and
efficiently recognise replay attacks.
