A denial-of-service vulnerability was found in Envoy's HTTP/2 implementation. The attack chains two techniques: an HPACK compression bomb that exploits header compression to amplify a single byte on the wire into a full header allocation on the server (repeated thousands of times per request), and a Slowloris-style hold using a zero-byte HTTP/2 flow-control window that prevents the server from ever freeing the allocated memory.

A remote attacker can exploit this in Envoy's default HTTP/2 configuration to consume and hold large amounts of server memory (up to 32GB in approximately 20 seconds), rendering the server inaccessible.

This vulnerability exists in each server's default HTTP/2 configuration and requires no authentication to exploit.

External Reference: https://blog.calif.io/p/codex-discovered-a-hidden-http2-bomb