The present disclosure relates to a security device, a system, and a method for securing a control apparatus. The security device includes a data security unit which is configured to secure data, data communication and information, and includes a first security component inside the data security unit to operate in a first operating mode, and at least one first monitoring unit to operate in a high-availability mode which, said first monitoring unit being configured to detect a fault present in the first security component. The high-availability mode is different from the first operating mode. The security device further includes a second security component which is configured to operate in the high-availability mode and to output a first response signal if a fault is detected by the first monitoring, where the high-availability mode is available independently from the first operating mode.