This indicates an attack attempt to exploit a Denial of Service Vulnerability in nginx HTTP Server.
A remote, unauthenticated attacker can exploit this vulnerability on a vulnerable version of Nginx server. By sending a continuous steam of HTTP/2 HEADERS and/or CONTINUATION frames, the attacker can cause a significant resource exhaustion on the target server, and leading to a potential denial-of-service condition especially if a distributed network of source machines are employed.
nginx HTTP Server prior to 1.16.1
nginx HTTP Server prior to 1.17.3
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.