The Java Remote Method Invocation, or Java RMI, is a mechanism that allows an object that exists in one Java virtual machine to access and call methods that are contained in another Java virtual machine; This is basically the same thing as a remote procedure call, but in an object-oriented paradigm instead of a procedural one, which allows for communication between Java programs that are not in the same address space.
One of the major advantages of RMI is the ability for remote objects to load new classes that aren't explicitly defined already, extending the behavior and functionality of an application. From here.
Default port: 1099, 1098
PORT STATE SERVICE REASON1099/tcp open rmiregistry syn-ack
The default configuration of rmiregistry allows loading classes from remote URLs, which can lead to remote code execution.
Basically this service could allow you to execute code.
msf> use auxiliary/scanner/misc/java_rmi_servermsf> use auxiliary/gather/java_rmi_registrynmap -sV --script "rmi-dumpregistry or rmi-vuln-classloader" -p <PORT> <IP>
msf> use exploit/multi/browser/java_rmi_connection_impl