Varnish Cache before 7.3.2 and 7.4.x before 7.4.3 (and before 6.0.13 LTS), and Varnish Enterprise 6 before 6.0.12r6, allows credits exhaustion for an HTTP/2 connection control flow window, aka a Broke Window Attack.
The product does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources.