This indicates an attack attempt to exploit a Remote Command Injection Vulnerability in Webmin.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when parsing HTTP requests. An authenticated remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Webmin version 1.910 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Currently we are unaware of any vendor supplied patch or updates available for this issue.