Information
A filename extension is a suffix added to a base filename that indicates the base filename's file format.
Rationale:
Visible filename extensions allow the user to identify the file type and the application it is associated with which leads to quick identification of misrepresented malicious files.
Impact:
The user of the system can open files of unknown or unexpected filetypes if the extension is not visible.
Solution
Perform the following to ensure file extensions are shown:
Graphical Method:
Open Finder
Select Finder in the Menu Bar
Select Preferences
Select Advanced
Set Show all filename extensions
Terminal Method:
Run the following command to enable displaying of file extensions:
$ sudo -u <username> /usr/bin/defaults write /Users/<username>/Library/Preferences/.GlobalPreferences.plist AppleShowAllExtensions -bool true
$ sudo killall Finder
example:
$ sudo -u seconduser /usr/bin/defaults write /Users/secondname/Library/Preferences/.GlobalPreferences.plist AppleShowAllExtensions -bool true
$ sudo killall Finder
Default Value:
Filename extensions are turned off by default.