I have a Arris TG1682G and just experienced this as well. Modem reset spontaneously and for ~10 - 15 minutes my leases (wired as well as wireless) were coming up with utopia.net as a DNS suffix and search domain. When I renewed the lease from my device, I received a 'standard' comcast.net DNS suffix and the expected DNS servers.
I talked with Customer Support who told me to contact Customer Security Assurance. Customer Security Assurance told me that "they don't deal with those devices (cable modems)" and they tried to send me somewhere else, but all that happened was I ended up on a "New Product line" IVR system that was asking about XI4 and then eventually disconnected me.
So yeah. I'm definitely not 'assured' about my security on Comcast.
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