Figured out why seemingly random Netgear Cable modems aren't supported by XFinity. (You'd think they'd put this ON their support portal, but here we are). There's a "fairly" new cable modem exploit called Cable Haunt and there are several Netgear modems listed as being vulnerable with no updates to fix the problem.
My modem (CM1000) has a new v2 model (CM1000v2) that IS supported by Comcast. I'm guessing (without saying) that the CM1000 is vulnerable to the exploit, and the v2 model fixes that issue. Don't know if the v1 model can get fixed with a firmware update, but given the lack of any information on netgear.com or xfinity.com, I'm guessing they don't want to point out the issue in order to stop people from being more nefarious?
Thought I'd post some messages. Doesn't look like the CM1200 is listed on cablehaunt.com, but there's a python script you can run on your network to test the vulnerability (will crash your modem if the vulnerability is present, You just restart to go back to "normal")
I sent a note to netgear to see what they say. Personally I'd consider upgrading. I know it sucks, but better than having your network hijacked for crypto mining or something. (Yes, it's a thing that happened to one of my Ubuntu servers that I traced to this issue.)
I could be wrong, but I don't think this is a DOCSIS 3.1 issue, it's a vulnerability with the "Spectrum Analyzer" built into the modems. Whatever that is.
