This question is about a capture-the-flag exercise. It's on-topic since the point of the exercise is to use various unix features. However, it's too localized in the sense that an answer would only be useful to solve this particular exercise. On the gripping hand, the answer would illustrate some simple unix penetration testing techniques. On the fourth hand (the dummy?), the answer is not discoverable because the question it's on isn't about any kind of repeatable, searchable problem.
I've answered, because, well, I might as well put up a solution somewhere, and illustrate these general concepts. But I am not at all convinced that the question should remain on the site. It could be generalized to ask about penetration testing in general, but then it would become far too broad: there are oodles of potential penetration avenues, this exercise just illustrates a couple.