This indicates an attack attempt to exploit a Denial of Service Vulnerability in Envoy.
A remote, unauthenticated attacker can exploit this vulnerability by sending many requests with a large number of small headers. Successful exploitation results in excessive CPU usage on the target system, leading to denial of service conditions.
Envoy Envoy 1.10.0 through 1.11.1
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.