If for instance directory was set to a variable and you did not check to make sure directory only contained valid data (or as most people say now make sure the data is not tainted) then it would be possible for people to try to inject other unsolicited behavior into your commands, such as adding additional directories. If you where doing system or exec this would be worse as they could actually add additional commands to be executed.

The main issue here is that cgi scripts can be called by a user and any thing that a user passes to your scripts canot be trusted without validation. The best thing you can do is to remember that and always hardcode variable results, set them equal to system variables, and/or taint check any user data for things that shouldn't be there.

This is just a short rant and I can provide much more of it if wanted.

Dave -- Saving the world one node at a time