This indicates an attack attempt against a remote code-execution vulnerability in the Microsoft .NET framework.
This issue is caused by a type-equality-checking error when the vulnerable software handles a malicious .NET application. It may allow remote attackers to execute arbitrary code by sending a crafted EXE file.
Microsoft .NET Framework 2.0
Microsoft .NET Framework 2.0 SP1
Microsoft .NET Framework 3.5
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's web site for the suggested workaround: