This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Citrix NetScaler.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Citrix NetScaler ADC 10.x and before 10.1-129.11
Citrix NetScaler Gateway 10.x and before 10.5-50.10
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.