According to KB887459, you can simply solve the issue by adding these lines to your Global.asax:
Sub Application_BeginRequest(Sender as Object, E as EventArgs)
If (Request.Path.IndexOf(chr(92)) >= 0 OR _
System.IO.Path.GetFullPath(Request.PhysicalPath) <> Request.PhysicalPath) then
Throw New HttpException(404, "Not Found")
End If
End Sub