A professor shares files with his students by placing them in a publicly accessible directory on the

Question:

A professor shares files with his students by placing them in a publicly accessible directory on the Computer Science department's Linux system. One day he realizes that a file placed there the previous day was left world-writable. He changes the permissions and verifies that the file is identical to his master copy. The next day he finds that the file has been changed. How could this have happened and how could it have been prevented?