Due to the REGISTER.ID function in Microsoft Excel 97 / 2000, remote execution of native code in a DLL file is possible under certain conditions. The REGISTER.ID function must refer to a specially formed DLL file containing the arbitrary code to be executed and it must reside on the local hard drive or on a UNC share. If a user opens a .xls file containing this function, the code specified will be run at the privilege level of the user. Excel and other MS Office files can be sent and automatically opened via browsers and HTML-aware email clients. In most cases, no prompt will be given to the user before Excel is started, and no warning will be given before the code is executed.
Microsoft has released the following patches which eliminates the vulnerability: