This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Pivotal Spring.
The vulnerability is due to insufficient sanitizing of HTTP requests in the application. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via crafted HTTP requests.
Spring Web Flow 2.4.0 to 2.4.4
Older unsupported versions are also affected
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.