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 a race condition on the target FTP server.
Vsftpd Vsftpd 1.2.1
Denial of service
Upgrade to the latest version of vsftpd (1.2.2 or later):