Hiding everything from ftpd user

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http://serverfault.com – i'm trying to figure out how to hide all files and folders from a specific ftp user on my system, for the sake of this question we call it "user123". one of my mobiles apps is uploading content into our ftp server, it means that the ftp credentials are hard coded into the app in a way that will danger the whole ftp account. this was taken care of in the new version, but due to compatibility issues some users are still using the old version. i cannot open up a new account jailed to that specific directory as it has a specific (again, hard coded path it has to follow). so i thought to myself (HowTos)