With Arma 3 version 1.50 the #include preprocessor command can read files from any number of parent directories up to root*. That way an unpacked mission could access any file stored on the machine.
This is a major security issue, because it's possible to copy stuff like memory dumps or browser passwords.

Below is an example on how to access and copy the "C:/Windows/system.ini" file.

*Apperently this only applies to unpacked missions in the documents folder. It should however always be impossible to access files outside of the game folders. This limitation seems to work for packed missions and addons, but not unpacked missions.

Edited. {F26872}