A method and system for secure cable modem initialization in a data-over-cable system is provided. The method includes sending a unique identifier, such an Internet Protocol ("IP") address and a selected time-value, such as an approximate message send time-value, in a registration request message. A message integrity check value is calculated using the unique identifier, the selected time-value and one or more configuration parameters in a pre-determined order. The message integrity check value is added to the registration request message. A cable modem termination system receives the registration request message and uses the message integrity check value to authenticate the message and determine if the registration request message was sent within a pre-determined period of time (e.g., 1 second) from a recognized cable modem. If not, the registration request message is discarded and a log file entry is added to a log file with information from the registration request message (e.g., network level and data-link level network addresses). The network address and selected time-value uniquely identify the cable modem and help prevent a rouge user from intercepting a valid cable modem registration request message and using it at a later time to register a rouge cable modem. The log file helps track rouge users attacking the data-over-cable system. The method and system provide improved security for registering cable modems in a data-over-cable system.