I have AT&T Gigapower with a Pace 5268AC, so not one of the modems discussed here.

I don't use its wifi and I have it configured for pass-thru mode. When I got service early this year, I briefly investigated bypassing it entirely. It turns out you need the modem to periodically respond to authentication packets from the AT&T network. But with some ingenuity, you can hang the router off an extra port on your own router and use it only for authentication purposes:

http://www.dslreports.com/forum/r29903721-AT-T-Residential-G...

I eventually decided not to do this because it's somewhat brittle and I didn't otherwise have any issues with the Pace. It's performance is fine.

But, given this disclosure, I'm going to revisit my decison. First, it seems like it's just a matter of time before the Pace has a similar security issue. Second, that kernel module for injecting HTTP advertisements. Just the idea of it bothers me.