This indicates an attack attempt against a Code Execution vulnerability in Microsoft Silverlight.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted Silverlight application. An attacker can exploit this by tricking an unsuspecting user into visiting a malicious webpage and execute arbitrary code within the context of the application.
Microsoft Silverlight 5
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.