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It depends on your server setup. Local exploits are more common than remote ones but the short answer is that it's more than likely that if he/she wanted to he/she could.

I sent u a PM to check out the flaws of this. If anyone else wants to take a crack at it let me know.

It's not at all easy to just check for an exploit. You might install something that changes it all.

Is it Linux? Probably.

If so your user will probably have the same username as you (by this I mean the server's usernames) and can do anything you can do.

So depending on what you have above that directory the user could view alter etc.

But it sounds like you are on shared hosting, local vulnerabilities already put you at almost the same risk if you are on a shared server.

Bottom line is that it's not something the average developer will be able to exploit but yes, if they can it is a risk.