DTOO132 - Excel - File Downloads must be configured for proper restrictions.
Disabling this setting allows websites to present file download prompts via code without the user specifically initiating the download. User preferences may also allow the download to occur without prompting or interacting with the user. Even if Internet Explorer prompts the user to accept the download, some websites abuse this functionality. Malicious websites may continually prompt users to download a file or present confusing dialog boxes to trick users into downloading or running a file. If the download occurs and it contains malicious code, the code could become active on user computers or the network.
Set the policy value for Computer Configuration -> Administrative Templates -> Microsoft Office 2010 (Machine) -> Security Settings -> IE Security 'Restrict File Download' to 'Enabled' and 'excel.exe' is checked.