Cable modems are maintained by the ISP, even if you own them. Otherwise they would not let you connect them to their cable. Otherwise people would be hacking their firmware to remove speed caps. The cable modem firmware can only be upgraded on a downlink from the ISP servers, using Simple Network Protocol over the cable. It's part of DOCSIS, and it keeps the cable companies in control of their own network. If you have a hackable modem and tinker with it they will ban you like Comcast did to all the kiddies who tinkered with their own parameters. I discovered this years ago when I needed a firmware update for my crippled linksys and was astonished that there was no way for a user to upgrade. And my ISP didn't even know this, either. "If that were true we would have to keep firmware upgrades for all brands of modems." D'oh! That's exactly correct.
That being said, a router is not necessarily a modem. Mine is separate. I own it and I and hacked my router firmware first week I had it. Technically, my router is just a linux computer attached to their modem, so they better not be changing my passwords.