This indicates an attack attempt against a Code Execution vulnerability in Microsoft .NET Framework.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. An attacker can trick an unsuspecting user into visiting a malicious webpage and execute arbitrary code within the security context of the target user.
.NET Framework 4 and .NET Framework 4 Client Profile
System Compromise: Remote attackers can gain control of vulnerable systems
Refer to the vendor's web site for the suggested workaround: