This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Ruby on Rail.
The vulnerability is due to an error when the vulnerable software in development mode handles a maliciously crafted HTTP request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
Rail version 188.8.131.52 and prior
Rail version 6.0.0.beta2 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the latest version from the vendor.