vsftpd 2.3.4 downloaded between 20110630 and 20110703 contains a backdoor which opens a shell on port 6200/tcp.
https://www.openwall.com/lists/oss-security/2011/07/11/5
https://vigilance.fr/vulnerability/vsftpd-backdoor-in-version-2-3-4-10805
https://security-tracker.debian.org/tracker/CVE-2011-2523
https://access.redhat.com/security/cve/cve-2011-2523
http://packetstormsecurity.com/files/162145/vsftpd-2.3.4-Backdoor-Command-Execution.html