In the method and the arrangement for checking the authenticity of a first communication subscriber in a communications network, a first fault information item is formed in the first communication subscriber using a fault detection data item of the first communication subscriber and an information item relating to a random data item. In a second communication subscriber in the communications network, a second fault information item is formed using a fault detection data item of the second communication subscriber and the information relating to the random data item. The authenticity of the first communication subscriber is checked using the first fault information and the second fault information.