This indicates an attack attempt against a code-execution vulnerability in the Microsoft .NET Framework.
The vulnerability is caused by an error in the Common Language Runtime (CLR) when it handles interfaces and delegations to virtual methods. It may allow remote attackers to execute arbitrary code by sending a crafted XBAP file, a crafted ASP.NET application, or a crafted .NET Framework application.
Microsoft .NET Framework 2.0 SP1, 2.0 SP2, 3.5, 3.5 SP1, and 3.5.1
Microsoft Silverlight 2 and 3 before 3.0.50611.0 on Windows
Microsoft Silverlight 2 and 3 before 3.0.41130.0 on Mac OS X
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's web site for the suggested workaround: