This indicates an attack attempt to exploit a Denial of Service Vulnerability in Apache Software Foundation Tomcat.
A remote, unauthenticated attacker can exploit this vulnerability by sending a large number of requests on several separate HTTP/2 connections with an initial window size of 0. Successful exploitation results in denial of service conditions on the target server.
Apache Software Foundation Tomcat 8.5.0 to 8.5.37
Apache Software Foundation Tomcat 9.0.0.M1 to 9.0.14
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.