It indicates an attacker attempted a Cross-Site Scripting attack against Microsoft ASP .NET. Microsoft has built in mechanisms in ASP.NET to prevent Cross-Site Scripting attacks. Unfortunately these security checks can be bypassed if a malicious attacker includes a null character in the beginning of a tag name. This could lead to the execution of arbitrary web script against the system.
Microsoft ASP.Net 1.1
Compromise of the affected system.
Apply appropriate patch from the vendor or Upgrade to non-vulnerable version if available.