Some FTP clients occasionally sends a invalid PORT number (PORT 0,0,0,0,0,0) (Tested with Python Ftplib and Windows WinInet API). The FtpServer server accepts the port number and starts processing the following STOR command. It sends 150 response. But during the call to openConnection inside STOR.execute( ) an exception is caught and the server responds with code 425. It is causing the FTP client to hang. It will perhaps be bettter if the server detects invalid port number and reject the STOR request with response 451.