The vulnerability stems from missing permission checks for the MLST command. In FTP protocol handling, command-specific methods like ftp_MLST are responsible for access control. The advisory explicitly states MLST didn't require 'l' permission, and FTP command handlers in pyftpdlib are typically implemented as methods prefixed with 'ftp_' in ftpserver.py. The fix would involve adding a permission check in this MLST handler method, which aligns with standard FTP command handling patterns in the library.