If you are referring to the certificate that is used by their secure public WiFi hotspots, you need to download the profile / app;
There are two different kinds of Xfinity Wifi that you can connect to:
The first is the one with the name “xfinitywifi”. After you select this one on your device, launch your browser. You will be redirected to the XFINTY WiFi sign in page. If the page doesn’t automatically redirect, type a different address in your web browser – such as www.xfinity.com – to be redirected to the XFINITY WiFi sign in page. You can then use your XFINITY username and password to sign in and start browsing, streaming and more.
The second one is named “XFINITY”. You cannot connect directly to this SSID, but you can get access by installing the security profile on your device. More information about the security profile is available here: https://wifi.xfinity.com/default.php#security
Once the application or profile is installed, sign in using your XFINITY username and password and start browsing, streaming and more.
More detailed information is available at this link: http://wifi.xfinity.com/faq.php
This no longer works and they have discontinued consumer access. Only business accounts can use the secure "XFINITY" hotspots now. The online documtation has not been updated to reflect business-only and is now incorrect.
Do you have any docs to corroborate this assertion ?