Mandiant has observed Russian nation-state attackers APT29 employing domain fronting techniques for stealthy backdoor access to victim environments for at least two years. There has been considerable discussion about domain fronting following the release of a paper detailing these techniques. Domain fronting provides outbound network connections that are indistinguishable from legitimate requests for popular websites.

https://www.fireeye.com/blog/threat-research/2017/03/apt29_domain_frontin.html