Earlier quoted context omitted.
So if I can spoof communication with that serial number on another device, I get free Internet. Same concept as MAC filtering not being really secure because I can just change MAC addresses in my packets.
How do you get the private key?
All of the popular embedded platforms have had scores of vulnerabilites - Qualcomm, Android, WebOS, etc. - patched over time, new ones found etc.
Heck, it even took Microsoft more than one try to start to get it right. An interesting story is Microsoft attempting to protect its first game platform--the original Xbox from the early 2000's. There were numerous security protections and all were bypassed - from encrypted boot code to a device-unique hard drive key stored in EEPROM.
Microsoft got better and smarter with the 360--this time with unique keys and eFuses in the CPU but it was still eventually bypassed--not after the effective lifetime of the platform though.