The attacker creates multiple request streams and continually shuffles the priority of the streams in a way that causes substantial churn to the priority tree. This can consume excess CPU, 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.