This indicates an attack attempt against a Buffer Overflow vulnerability in Trellian FTP clients and LeapFTP FTP client.
The vulnerability is due to an improper boundary check condition in the application when handling a malicious PASV response packet. An attacker can exploit this by tricking an unsuspecting user into visiting a malicious server and execute arbitrary code within the context of the targeted user.
Trellian FTP client 3.1
LeapFTP FTP client 188.8.131.520
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's Web site for suggested workaround.