This indicates an attack attempt against a remote code-execution vulnerability in the Microsoft .NET Framework.
The vulnerability is caused by the an error when the vulnerable software handles malformed .NET verifiable code. An attacker may exploit this to execute arbitrary code via a crafted XAML browser application (XBAP), a crafted ASP.NET application, or a crafted .NET Framework application.
Microsoft .NET Framework 1.0 SP3
Microsoft .NET Framework 1.1 SP1
Microsoft .NET Framework 2.0 SP1
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the patch, available from the vendor's website: