Apache Tomcat servers released in the last 13 years are vulnerable to a bug named Ghostcat that can allow hackers to takeover unpatched systems.
Discovered by Chinese cybersecurity firm Chaitin Tech, Ghostcat is a flaw in the Tomcat AJP protocol.
AJP stands for Apache JServ Protocol and is a performance-optimized version of the HTTP protocol in binary format. Tomcat uses AJP to exchange data with nearby Apache HTTPD web servers or other Tomcat instances.
Tomcat's AJP connector is enabled by default on all Tomcat servers and listens on the server's port 8009.