This indicates an attempt to exploit a buffer overflow vulnerability in some File Transfer Protocol (FTP) servers.
The vulnerability is due to inadequate user input validation of the "USER" command. A remote attacker can cause Denial of Service (DoS) on a vulnerable system by sending it a specially crafted FTP command.
Affected Products
Shadow Op Software Dragon Server versions 1.0 and 2.0 BlackMoon FTP Server versions prior to and including 1.5 FTP servers from other vendors are potentially vulnerable as well.
Recommended Actions
Apply the appropriate patches or upgrade the system to the latest non-vulnerable version.