In Vinyl Cache before 9.0.1 and Varnish Cache before 9.0.3, a deficiency in HTTP/2 request parsing can be exploited to launch a backend request desync
attack (request smuggling), which in turn can be used for cache poisoning,
authentication bypass, or possibly even information disclosure and manipulation. The attack vector only exists if HTTP/2 support is enabled by setting the
feature parameter to contain +http2. HTTP/2 support is disabled by
default.