The attacker opens a number of streams and sends an invalid request over each stream that should solicit a stream of RST_STREAM frames from the peer. Depending on how the peer queues the RST_STREAM frames, this can consume excess memory, CPU, or both, potentially leading to a denial of service.

See https://groups.google.com/g/envoy-announce/c/ZLchtraPYVk, https://www.envoyproxy.io/docs/envoy/v1.11.1/intro/version_history and https://github.com/Netflix/security-bulletins/blob/master/advisories/third-party/2019-002.md.