A serial communication system for communicating data over a Controller Area Network (CAN) bus comprises a security slave device located between a first system node and a Controller Area Network (CAN) bus. The system is characterised in that: said security slave device further comprises a tagging means for inserting data indicative of said first node into a Controller Area Network (CAN) frame received from said first node; said system further comprises a security master device, located between said Controller Area Network (CAN) bus and a second system node; said security master device further comprises a means of extracting said data indicative of first said node from a received data frame; and said system further comprises a means of checking the validity of received Controller Area Network (CAN) frame associated to said extracted data indicative of said first node.