Disable OS/2 and/or Posix subsystems

It is a good security policy to disable optional, unused sub-systems Microsoft
included with Windows – NT, W2K, & XP. To disable the obsolete and insecure
posix and os/2 subsystems by clearing the multistring value named “Optional” in the Registry entry:

Key: SYSTEM\CurrentControlSet\Control\Session

Under some OSs the string with have both “os/2” and “posix” strings. XP
generally just has “posix”. In any case, clear the value.

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