This indicates an attack attempt to exploit a Denial of Service Vulnerability in ProFTPD Project ProFTPD.
A remote, unauthenticated attacker can exploit this vulnerability by sending a maliciously long FTP command. Successful exploitation will cause the vulnerable server to use up a single CPU core. If ProFTPD is run in a multi-threading mode, an attacker can send several such requests to use up all cores at 100%, leading to a denial of service condition.
ProFTPD Project ProFTPD prior to 1.3.6b
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.