The session hijacking attack targets the application layers control mechanism, which manages authenticated sessions between a host PC and a PLC. During such sessions, a session key is utilized to maintain security. However, if an attacker captures this session key, they can inject traffic into an ongoing authenticated session. To successfully achieve this, the attacker also needs to spoof both the IP address and MAC address of the originating host which is typical of a session-based attack.
A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes).