Authentication device for a vehicle, wherein the vehicle has a communication network for communicating communication data, comprising: a communication interface , which is connectable to the communication network of the vehicle, wherein the communication interface is configured to receive the communication data; and a processor configured to detect a characteristic of the received communication data, wherein the processor is configured to authenticate the communication data on the basis of a comparison of the detected characteristic with a reference characteristic and to detect an authenticity of the communication data if the characteristic of the communication data corresponds to the reference characteristic, and to detect a missing authenticity of the communication data if the characteristic of the communication data differs from the reference characteristic, wherein the processor is furthermore configured to detect a receiver of the communication data in the communication network and to control the communication interface in response to the detection of the authenticity of the communication data for forwarding the communication data to the receiver, and wherein the processor is configured to send a warning message or an error message in response to the detection of the missing authenticity of the communication data, characterized in that the processor is configured to detect a degree of the deviation of the characteristic from the reference characteristic, wherein the processor is configured to send the error message if the degree of the deviation exceeds a first threshold value, and to send a warning message if the degree of the deviation falls below the first threshold value and exceeds a second threshold value. a communication interface , which is connectable to the communication network of the vehicle, wherein the communication interface is configured to receive the communication data; and a processor configured to detect a characteristic of the received communication data, wherein the processor is configured to authenticate the communication data on the basis of a comparison of the detected characteristic with a reference characteristic and to detect an authenticity of the communication data if the characteristic of the communication data corresponds to the reference characteristic, and to detect a missing authenticity of the communication data if the characteristic of the communication data differs from the reference characteristic, wherein the processor is furthermore configured to detect a receiver of the communication data in the communication network and to control the communication interface in response to the detection of the authenticity of the communication data for forwarding the communication data to the receiver, and wherein the processor is configured to send a warning message or an error message in response to the detection of the missing authenticity of the communication data, characterized in that the processor is configured to detect a degree of the deviation of the characteristic from the reference characteristic, wherein the processor is configured to send the error message if the degree of the deviation exceeds a first threshold value, and to send a warning message if the degree of the deviation falls below the first threshold value and exceeds a second threshold value. <EOS>