A security device includes an interface and a processor. The interface is configured for connecting to a bus that serves a host device and a non-volatile memory (NVM) device. The processor is connected to the bus in addition to the host device and the NVM device. The processor is configured to detect on the bus a boot process, in which the host device retrieves boot code from the NVM device, and to ascertain a security of the boot process, based on an authentic copy of at least part of the boot code of the host device.