Microsoft's Excel team has made a change in the behavior of certain file types to increase security. This change came in the security updates KB3115262, KB3170008, and KB3115322.
You can follow the steps below to change your settings. Please note that the change needs to be done for each user that works with the Excel export.
Information from Microsoft
Previously, when you tried to open an HTML or XLA file with an .XLS file extension from an untrusted location, Excel would warn about the mismatch between the file extension and content, but would still open the workbook without Protected View security. After the security updates Excel no longer will open the workbook because these files are not compatible with Protected View and there is no warning or other indication it was not opened. We apologize that Excel is showing a blank screen instead of a more helpful error message with information about what to do next.
We have a few options for workarounds. These are in order from safest to riskiest. While some people in the forums have suggested rolling back the security patch, we do not recommend that option as it can leave you open to other current and future threats.
- The best option is to move away from using HTML wrapped as .xls. If you use native formats (e.g. xls, xlsx, xlsb) which will open in protected view when untrusted, this will provide some level of protection from the documents being opened.
- You can unblock access for individual files you know are safe. To do this:
- Right click on the file and choose Properties
- On the General tab, click Unblock
- Click OK
- You can make use of existing Trusted Locations capabilities in Excel 2010, 2013, and 2016 via File -> options -> Trust Center -> Trust Center Settings -> Trusted Locations.
- You can save the web html file to a trusted location on the local machine (Excel comes with a set of default trust locations). If you do not see the local folder location you trust for these files, then press “Add new location…” button and add it in the Trusted Location dialog. If the HTML document is in a trusted location the KB fix is not applied (e.g. the unsafe HTML file is not blocked).
This approach may unblock you, but it carries some risk as files of any file type in Trusted Locations are fully trusted. If an attacker can drop files into the trusted location they can easily exploit users who open such documents. Be especially cautious when specifying a custom folder as a trusted location.
You can read more information in this article.