The HTTP/2 protocol does not consider the role of the TCP congestion window in providing information about content length, which makes it easier for remote attackers to obtain cleartext data by leveraging a web-browser configuration in which third-party cookies are sent, aka a "HEIST" attack.
https://tom.vg/papers/heist_blackhat2016.pdf
http://www.securitytracker.com/id/1036746
http://www.securitytracker.com/id/1036745
http://www.securitytracker.com/id/1036744
http://www.securitytracker.com/id/1036743
http://www.securitytracker.com/id/1036742
http://www.securitytracker.com/id/1036741