This indicates an attack attempt to exploit a Denial of Service vulnerability in Microsoft .NET implementation of XML Digital Signatures.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the .Net application when handling crafted XML file. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
Microsoft .NET Framework 2.0 Service Pack 2
Microsoft .NET Framework 3.5
Microsoft .NET Framework 3.5 Service Pack 1
Microsoft .NET Framework 4
Microsoft .NET Framework 4.5
Denial of Service: Remote attackers can crash vulnerable systems or services
Apply the most recent upgrade or patch from the vendor.