This indicates an attack attempt against a Denial-Of-Service vulnerability in vsftpd.
The vulnerability is caused by an error when the vulnerable software handles a large number of incoming TCP connections. It allows a remote attacker to cause a race condition error on a targeted FTP server.
Vsftpd Vsftpd 1.2.1
Denial of Service: Remote attackers can crash vulnerable systems.
Upgrade to the latest version of vsftpd (1.2.2 or later):