http://www.percya.com/2016/08/chinese-ca-wosign-faces-revocation.html
One of the largest Chinese root certificate authority WoSign issued many fake certificates due to an vulnerability. WoSign's free certificate service allowed its users to get a certificate for the base domain if they were able to prove control of a subdomain. This means that if you can control a subdomain of a major website, say percy.github.io, you're able to obtain a certificate by WoSign for github.io, taking control over the entire domain.
