Title

File types must be configured to provide mismatch warnings (Cat II impact)

Discussion

Excel can load files with extensions that do not match the files' type. For example, if a comma-separated values (CSV) file named example.csv is renamed example.xls, Excel can properly load it as a CSV file. Some attacks target specific file formats. If Excel is allowed to load files with extensions that do not match their file types, a malicious individual can deceive users into loading dangerous files that have incorrect extensions. By default, if users attempt to open files with the wrong extension, Excel opens the file and displays a warning that the file type is not what Excel expected.

Check Content

Verify the policy value for User Configuration -> Administrative Templates -> Microsoft Excel 2013 -> Excel Options -> Security "Force file extension to match file type" is set to "Enabled (Allow different, but warn)". Procedure: Use the Windows Registry Editor to navigate to the following key: HKCU\Software\Policies\Microsoft\Office\15.0\excel\security Criteria: If the value ExtensionHardening is REG_DWORD = 1, this is not a finding.

Fix Text

Set the policy value for User Configuration -> Administrative Templates -> Microsoft Excel 2013 -> Excel Options -> Security "Force file extension to match file type" to "Enabled (Allow different, but warn)".

Additional Identifiers

Rule ID: SV-53811r1_rule

Vulnerability ID: V-17621

Group Title: DTOO143 - Force File Extension to match type

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.