The present invention teaches methods and systems for blocking unauthorized access to cable modem configuration files stored on trivial file transfer protocol (TFTP) servers. Filenames are modified by the DHCP to incorporate an authentication key (and optional cloaking) prior to transmission to the cable modem. When the TFTP server receives a modified filename, it also generates an authentication key. The authentication keys must match in order for the cable modem to receive the configuration file requested. At a minimum, authentication keys depend upon the un-modified filename, the cable modem IP address and a "coordination pass phrase" known to the TFTP server and DHCP server, but not known to the cable modem. Variations include optional cloaking, various actions performed for non-matching authentication keys, selection of authentication key generating algorithm and inclusion of cable modem MAC address in the authentication key for all cable modems or for premium service customer cable modems.