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 extensively use Bash as their default shell, making them highly susceptible to the Shellshock vulnerability.

Unix-like operating systems, the broader category that includes Linux, generally rely on Bash or other Bourne-again shell derivatives, and were therefore impacted.

OS X (now macOS) includes Bash and was affected by the Shellshock vulnerability, particularly for services or scripts that invoked Bash.

The Shellshock vulnerability (CVE-2014-6271, among others) exploited a flaw in the Bash shell, a command-line interpreter widely used on Unix-like operating systems. Since Windows operating systems do not natively use Bash as their primary shell, they were not directly affected by this specific vulnerability.