This indicates an attack attempt to exploit a Denial of Service vulnerability in Apache Tomcat.
A remote, unauthenticated attacker can exploit this vulnerability by sending a large number of requests and not sending any WINDOW_UPDATE frames with a Stream ID of 0. Successful exploitation results in denial of service conditions on the target server.
Apache Software Foundation Tomcat 8.x prior to 8.5.41
Apache Software Foundation Tomcat 9.x prior to 9.0.20
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.