The ftp_QUIT function in ftpserver.py in pyftpdlib before 0.5.0 allows remote authenticated users to cause a denial of service (file descriptor exhaustion and daemon outage) by sending a QUIT command during a disallowed data-transfer attempt.
Improper Input Validation The product receives input or data, but it does
not validate or incorrectly validates that the input has the
properties that are required to process the data safely and
correctly.