[MertsA]

At least it appears to be based on the serial number. Only using the last 5 is still pretty bad though but plenty of cable modems treat the serial number as privileged information. It's already a password essentially for SNMP access provided that your ISP hasn't blocked access to it.

[esseye]

It is exactly as privileged as going to the website http://192.168.100.1 and clicking HW/FW versions, which proudly displays the complete serial for you. There is no authentication of any sort and it is not encrypted at any point.