This indicates an attack attempt to exploit a Security Bypass vulnerability in Microsoft .Net Framework.
The vulnerability is due to lack of ASLR security feature in the the vulnerable DLL. An attacker can use this vulnerability to bypass security features the vulnerable machine leading to other attacks.
Microsoft .NET Framework 2.0 Service Pack 2
Microsoft .NET Framework 3.5
Microsoft .NET Framework 3.5.1
Security Bypass: Remote attackers can bypass security checks of vulnerable systems
Apply the most recent upgrade or patch from the vendor.