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Cannot Open Externally-Generated XLS File

If you are exporting to Excel from a third-party platform such as Salesforce or Planisware, you may find that you have issues opening files with an *.xls extension. This is because some platforms save their Excel files in an internal XLSX format, but create files with an external XLS (no X) extension. This mismatch between format and extension causes Excel to generate a security warning, which can sometimes freeze Excel.

To remedy this issue, there are several options:

Launch Excel Before Opening the File

Launch Excel first and then go to File > Open to open the XLS file. You may still get the following warning, but can dismiss it more easily:

Save your Export in a Different Format

CSV and XLSX file formats do not fall victim to the same security warnings from Excel. So, if you export to CSV or XLSX instead of to XLS, you may have better luck. Alternatively, you can manually change the extension of your exported file from XLS to XLSX before opening it, and it should pass Microsoft's security screening.

Disable Excel Extension Hardening

A more robust approach is to disable Excel's validation of XLS vs. XLSX formats. To do this:

  1. Open your Registry (Start > Run > regedit.exe)
  2. Navigate to HKEY_CURRENT_USER\SOFTWARE\MICROSOFT\OFFICE\16.0\EXCEL\SECURITY (Your version number will vary based on your year of Excel. Excel 2016, Excel 2019, Excel 2021, and Office 365 are all 16.0)
  3. Right click in the right window and choose New > DWORD
  4. Type "ExtensionHardening" as the name (without the quotes) if it's not already listed.
  5. Verify that the data has the value "0" (again, without quotes):

This option is the most robust, but does come with a few caveats:

  • Excel has a tendency to undo this setting. So when you install Excel service packs or other periodic updates, you may find that you need to re-enable this setting by changing the registry value back from 1 to 0.
  • Excel extension hardening, while considered a nuisance by many, does serve an important purpose as it relates to information security. If you choose to disable extension hardening, please exercise caution when opening Excel files from other people and ensure that your virus protection software is up-to-date and is set to scan all files before you download or open them.


Last Updated: August 18, 2021