ftpserver.py in pyftpdlib before 0.5.2 does not require the l permission for the MLST command, which allows remote authenticated users to bypass intended access restrictions and list the root directory via an FTP session.
http://code.google.com/p/pyftpdlib/source/detail?r=596
http://code.google.com/p/pyftpdlib/source/browse/trunk/HISTORY