This indicates an attack attempt to exploit a remote Code Execution vulnerability in Ruby on Rails.
The vulnerability is due to a flaw in Action Pack in Ruby on Rails when handling a crafted HTTP request. A remote attacker can exploit this to execute arbitrary code execution within the context of the application, via a crafted HTTP request.
Ruby on Rails 3.2.x before 18.104.22.168,
Ruby on Rails 4.x before 22.214.171.124
Ruby on Rails 4.2.x before 126.96.36.199
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's website for patch or updates.