Shellshock allowed an unauthorized user to gain access to a server. It affected many Internet- facing services, which OS did it not directly affect?

Linux systems use Bash as a core default shell and were severely and directly impacted by Shellshock.

Many Unix variants ship with or support Bash and were confirmed vulnerable to the exploit.

OS X used Bash as its default login shell at the time of disclosure and was directly vulnerable.

Windows relies on cmd.exe and PowerShell as its native shells and does not ship with GNU Bash. Because Shellshock triggered through malformed environment variables passed to Bash during shell invocation, Windows systems had no native exposure to the flaw. Exposure on Windows would only occur if a third-party Bash environment such as Cygwin was explicitly installed and configured.