Title

Force file extension to match file type created - Excel (Cat II impact)

Discussion

Excel 2007 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 person 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

The policy value for User Configuration -> Administrative Templates -> Microsoft Office Excel 2007 -> Excel Options -> Security “Force file extension to match file type” will be set to “Enabled (Allow different, but warn)”. Procedure: Use the Windows Registry Editor to navigate to the following key: HKCU\Software\Policies\Microsoft\Office\12.0\Excel\Security Criteria: If the value ExtensionHardening is REG_DWORD = 1, this is not a finding.

Fix Text

The policy value for User Configuration -> Administrative Templates -> Microsoft Office Excel 2007 -> Excel Options -> Security “Force file extension to match file type” will be set to “Enabled (Allow different, but warn)”.

Additional Identifiers

Rule ID: SV-18762r1_rule

Vulnerability ID: V-17621

Group Title:

Expert Comments

CCIs tied to check.

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