The attacker sends continual pings to an HTTP/2 peer, causing the peer to build an internal queue of responses. Depending on how efficiently this data is queued, this can consume excess CPU, memory, 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.