adding ChrootDirectory to sshd_config to jail a user prevents the user to login

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http://serverfault.com – i've created an user on ubuntu 10.04 and wanted to jail the user by disabling ssh shell and allow sftp. when there is no ChrootDirectory in the sshd_config the user can login just that it can see other sibling folders. let's say i only want the user to access /var/www/vhost/shop.mydomain.com so that's the home folder of my new user is set to that.but the use could browse via filezilla everything under /var/www/vhost/*. i followed this post and so far that's the only kink. I would be glad if anyone can give me a tip. thanks (HowTos)