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.
"https://github.com/pypa/advisory-database/blob/main/vulns/pyftpdlib/PYSEC-2010-6.yaml"