This indicates an attack attempt to exploit a Remote Command Execution vulnerability in Cesanta Mongoose.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Cesanta Mongoose 6.8
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor: