The HTTP/2 implementation accepted streams with excessive numbers of SETTINGS frames and also permitted clients to keep streams open without reading/writing request/response data. By keeping streams open for requests that utilised the Servlet API’s blocking I/O, clients were able to cause server-side threads to block eventually leading to thread exhaustion and a DoS.
Denial of Service
Apache Tomcat 8.x
Apache Tomcat 9.x
Update to version 9.0.16 or 8.5.38.