The invention relates to an electrical system of a motor vehicle with control apparatuses, which communicate with one another by means of a data bus. To recognize manipulations to the electrical system of a motor vehicle, in particular on the software of the control apparatuses of the electrical system, and to derive suitable measures, it is proposed that a master security module is provided in a first control apparatus and a client security module is provided in each case in a plurality of the further second control apparatuses, and the master security module of the first control apparatus, preferably a central gateway control apparatus, signs a message and sends the signed message to at least one of the second control apparatuses by means of the data bus. The client security module of the second control apparatus checks the signed message received from the master security module as to whether it comes from an authorized master security module.