I had the same problem and simply logged onto the gateway via the xfinity mobile app, reset the SSID and password and then log on to your desktop or laptop computer, open the Windows Registry, search/find "utopia.net", right click on it and delete. Then, make sure you 'edit' and 'find next' to be sure and get rid of all traces.
You can actually delete it from the registry first if you want and then reset the gateway SSID and password. This worked like a charm for me and haven't had any problems since. Calling comcast is a waste of time as they will do nothing nor admit that this is an issue with their modems. Their techs are generally trained to troubleshoot connection problems, and not malware problems. Also, I highly doubt there are any anti-malware programs out there that will fix it because it's not your normal malware/hijack.
Hope this helps.
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Same thing happened to me....we were having downstream/upstream issues and Comcast came out and gave us a new modem. For the first few weeks, no problems (it could have been there and I just didn't notice it). Then, our internet became intermittent on a daily basis. On and off, on and off. Or, the internet would be extremely slow. I decided to logon to the Gateway to see if maybe someone else was hijacking our wifi. This is where I first noticed the issue. We had several devices connected (via MoCa) that were unknown and I had no idea who they were or what MoCa was. So, I began googling this issue and read about the utopia.net hijack. I ran cmd.exe as administrator and ran ipconfig/all and sure enough, the hsd1.comcast.net was not there and utopia.net was. Here's how I fixed it: Simply log on to the xfinity mobile app, click on 'internet' and then your device (gateway), then tap 'Advanced Settings' to log on to the gateway via 'admin tool'. Once there, you can reset your SSID and password. This will also reset the gateway to factory default. That seemed to fix the issue. Also, I logged on to each computer in our house connected to the internet and cleared 'Utopia.net' from the Windows Registry. Haven't had any problems since and it hasn't came back!
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