This indicates an attack attempt to exploit a Denial of Service Vulnerability in Apache Software Foundation Tomcat.
The vulnerability is due to a failure to release the HTTP 1.1 processor after initiating a direct h2c connection.
A remote, unauthenticated attacker can exploit this vulnerability by initiating a large number of h2c connections to a vulnerable server. Successful exploitation results in the consumption of excessive amounts of memory, eventually leading to denial of service conditions.
Apache Software Foundation Tomcat 8.5.0 to 8.5.56
Apache Software Foundation Tomcat 9.0.0.M1 to 9.0.36
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.