You need to use your Comcast account credentials to use the open xfinitywifi hotspots.
The default wireless password is the one found on the gateway, that is for your private home wireless network.
Also, you can enable/disable the xfinitywifi hotspot and manage any of your devices that connect to it, in your Comcast account settings. If you want it on and it's working probably best to leave it now.
My xfinitywifi SSID has been disabled for months, but thought it did NOT require a password.
To automatically connect at Xfinity WiFi Hotspots and get the best possible experience, install the security profile on your device. To sign in manually when you are at a hotspot:
Note: Once you've signed in and registered your device, you may be prompted to install the Xfinity WiFi secure profile, which enables you to take advantage of enhanced security features at many hotspot locations. The installation process varies by operating system, so please follow the on-screen prompts.