The disclosure relates to a data processing apparatus. The data processing apparatus may comprise a memory storing a candidate service level response to an intrusion to an operating system having a plurality of operating system services. The data processing apparatus may comprise processing circuitry coupled to the memory. The data processing apparatus may comprise an output coupled to the processing circuitry. It may be that the processing circuitry is to, depending on an alert indicative of the intrusion: select from the memory, for an operating system service of the said plurality of operating systems, the said operating system service being related to the alert, the candidate service level response to the intrusion; and provide a signal to the output depending on the candidate service level response selected in respect of the said operating system service.