This indicates an attack attempt to exploit a remote Code Execution vulnerability in Metasploit RPC server.
The vulnerability is due to improper validation of an HTTP request. A remote attacker may be able to exploit this to execute arbitrary codes within the context of the process, via a crafted HTTP request.
Metasploit 4.15 and prior version
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.
https://github.com/rapid7/metasploit-framework or use the command msfupdate to update to the latest version