Shellshock had the potential for an unauthorized user to gain access to a server. it affected many internet-facing services, which os did it not directly affect?

Windows

Explanation:

ShellShock: Also called Bashdoor, it is a security bug which is found in the Bash shell of operating system e.g. Unix. It allows the hacker to execute scripts and commands by gaining unauthorized access. This affects many machines, devices that use Bash. Servers like web servers are most affected as most of them run Unix based OS like OS X. Most vulnerable are those who use Unix or Linux operating systems.  

Microsoft Windows does not use Bash so it does not directly affect Windows os. For those who are using Windows PC, Windows phones and websites that are developed by MS software are non-vulnerable to ShellShock.